Relay Fundamentals

Open an HVAC control panel, and you’re likely to find at least one relay mounted inside. Sometimes you find lots of relays! Even though relay technology has been around since the 1800s, there is still a need for relays in control panels.

What components make up a relay? How are relays typically shown in wiring diagrams? What functions can a relay perform? Stay tuned for some answers…

What Components Make Up A Relay?

A relay is constructed from the following components:

  • Electromagnet (coil of wire with metal core)
  • Armature (arm that moves when attracted by the energized electromagnet)
  • Spring (keeps the armature retracted when the electromagnet is de-energized)
  • Contact(s) (one or more electrical contacts that open or close when the armature moves)
  • Electrical Terminals (so you can make connections to the electromagnet and contacts)

How Does It Work?

You might have built an electromagnet in high school science class and used it to pick up small metal objects.  An electromagnet is a coil of wire wound around a metal core.  When the coil of wire is hooked to an electrical power source and current flows through the coil, a magnetic field is produced which surrounds the coil and the core:

While the electromagnet is energized, any metal objects which come close will be physically pulled towards core.

When the switch is opened, the current flow stops and the magnetic field disappears.  When the magnetic field disappears, nearby metal objects are no longer attracted to the core:

We now place a pivoted metal arm (armature) above the electromagnet, and we connect a spring to keep the armature pulled away from the electromagnet under “normal” (electromagnet de-energized) conditions:

If we now energize the coil, the magnetic field will overcome the spring force and pull the armature down:

So… we have an armature which we can move up and down by de-energizing or energizing the electromagnet coil.  What do we do with it?

We use the moving armature to open and close electrical contacts which are separate from the coil circuit.  We can do it like this:

In this example, we’re using the metal armature itself as the “Common” or “Wiper” for a double-throw switch.  The tip of the armature makes contact with one of two possible points depending on the position of the armature.

When the relay is de-energized, the armature is up and the wiper makes connection with the “Normally Closed” (NC) contact.

When the relay is energized, the armature is down and the wiper makes connection with the “Normally Open” (NO) relay contact:

This example relay would be known as a “Single Pole, Double Throw” (SPDT) or “1 Pole, Double Throw” (1PDT) relay.  It has one set of contacts (1 Pole) with both Normally Open and Normally Closed (Double Throw) connections.

Relays can have more than one set of contacts (poles).  In the next example, the metal armature is not part of the electrical circuit but instead moves a plastic connector that activates two sets of contacts:

This would be known as a “Double Pole, Double Throw” (DPDT) or “2 Pole, Double Throw” (2PDT) relay.

The number of poles (contact sets) can be expanded (3PDT relays, 4PDT relays etc.).  Typically 4 poles is the largest relay we would see in an HVAC control panel.  If you need more than 4 poles in a relay, you would probably use two relays with the coils wired together in parallel.  Shown below are a DPDT relay and a 4PDT relay wired together to create 6PDT relay action:


Electrical Ratings of a Relay

The coil of a “classic” relay (no electronics) is typically rated for a specific voltage and either AC or DC operation but not both.

There are some modern relays (like Functional Devices “Relay In A Box”) that contain electronic circuits that allow the coil to operate over a range of voltages and either AC or DC operation.

If in doubt, the relay’s datasheet should clearly spell out the acceptable voltage range for the coil and whether it’s AC only, DC only, or AC/DC compatible.

Relay coils are generally tolerant of some variation in the coil voltage, a typical datasheet specification is 80% – 120% of rated coil voltage.  So if a 24V transformer is “running a little hot” (putting out maybe 27V) or there is a bit of voltage drop in the coil connecting wires (maybe the coil is only getting 22V) the relay will still function just fine.

The relay contacts will have ratings for the maximum voltage allowed and the maximum current allowed.  Sometimes there are multiple ratings depending on the type of load being switched (for a motor load sometimes the contacts are rated in horsepower for example).  The circuit being switched by the contacts should not put more voltage or current on the contacts than allowed by the datasheet.

Relay Symbols Used in Wiring Diagrams

The relay coil may be drawn several different ways:

The relay contacts are typically shown one of two ways:

Relays go by several possible names on an electrical drawing:

‘R’       (not a great idea since resistors can have this designation too)
‘RL’     (better)
‘RLY’   (better still)
‘K’      (Where did this come from?  We don’t know.  But it’s widely used!)

If there are multiple relays, the name will be followed by a number or a letter as in RLY1, RLY2, RLY3 or RLYA, RLYB, RLYC.  There might be a hyphen (-) between the name and number or letter too.  There is no specific standard on how relays must be named.

Sometimes the relay is shown as a single assembly with coil and contacts arranged in a rectangle:

But sometimes the coil and the contacts are scattered around the drawing:

Relay Terminal Numbering

Unfortunately there does not seem to be a standard for the terminal numbering on relays.

Most of the relays we use in HVAC panels plug into a base socket.  The pins/blades of the relay have assigned numbers and then the bases they plug into have assigned numbers on the screw terminals.  If the same brand of relay and base are used together, the blade numbers usually match the terminal numbers on the base.  If, however, different brands of relay and base are used together, the blade numbers may not match the base terminal numbers even though the two parts are physically compatible (example:  IDEC SH3B-05 relay plugged into Omron PTF11A base).  It’s a good idea to use the same brand of relay and base to avoid confusion.

Some electronic relays such as Functional Devices “Relay In A Box” have color-coded flying leads rather than screw terminals.  Consult the device datasheet to determine the function of each color-coded lead.

Functions a Relay Can Perform

So what useful functions can a relay perform?

  • Use a low voltage and/or low current signal (on the coil) to switch a large voltage and/or large current circuit (on the contacts).
  • Use a DC signal (on the coil) to switch an AC circuit (on the contacts) or vice-versa.
  • Use one signal (on the coil) to switch multiple circuits (multi-pole contacts).
  • Invert the sense of a signal (using the Normally Closed contact).
  • Use a signal from one circuit (on the coil) to switch another circuit (on the contacts) where the two circuits are not allowed to have any direct electrical connection.
  • Use one signal (on the coil) to switch between two alternate loads (on the contacts).
  • Various combinations of the above.

The example below demonstrates a relay performing a combination of several of the above functions.  A low voltage, low current control signal from a controller is switching a high voltage, high current load on and off.  The relay is also allowing a DC control signal to switch an AC load.  There is also electrical isolation between the drive circuit and the load circuit so that the controller is protected from the high voltage 240V power source:

In the next scenario a smoke detector signal is being sent to three different systems simultaneously while maintaining isolation between the systems to make sure there is no unintended interaction between the systems:

Here is an application where we want to reverse the sense of an electrical signal.  We have a foil alarm loop glued to a glass door.  When the foil is intact (current flowing through foil) we want to hold off the Alarm light and horn.  When the foil is broken (current flow stops) we want to energize the Alarm light and horn.  We can get the reverse action we want by using the Normally Closed contact on the relay:

Let’s see how we can use both the N.O. and N.C. contacts to get “alternate action” between two loads.  We decide that in addition to the ALARM light, we also want a NORMAL light for our alarm circuit that is on when the foil connection is intact.  So we want the action of the NORMAL and ALARM lights to alternate, one or the other is on at any time, but never both at the same time.  We can get this alternate action by adding the NORMAL light to the N.O. relay contact:

You may begin to see how versatile relays are.  Even with all the high-tech electronics in control panels, relays will be with us for a long time to come.  The operation is easy to understand and they can be used for many different applications.

Magnetic Latching Relays

There is a special class of relays known as “magnetic latching relays.”  These relays contain one or more small permanent magnets which hold the armature in the last commanded position when the coil drive is removed.  When a coil drive signal is applied, the magnetic field from the coil is stronger than the permanent magnets and can thus move the armature to the opposite position in spite of the permanent magnet’s presence.

Magnetic latching relays must have two coil drive commands, Latch and Unlatch.  Some relays accomplish this with two separate coil windings and two command leads:

Other magnetic latching relays have just two coil wires, but the polarity of the applied DC drive determines whether the command is Latch or Unlatch:

The Latch and Unlatch drive signals are typically meant to only be momentary, you normally would not leave either signal on the coil continuously.

Magnetic latching relays are frequently used in lighting circuits.  One example is the popular General Electric RR7 lighting relay.


Some points to remember about relays:

  • Make sure that the relay coil voltage and AC/DC rating matches the coil drive signal in the application circuit.
  • Some relays are available with a “coil energized” light built in. If any troubleshooting has to be done on the panel, these lights can be very helpful.
  • Some relays are available with a manual override button built in. If any troubleshooting has to be done on the panel, these override buttons can be very helpful.
  • Make sure that the relay contact voltage and current ratings are at least as high as the volts and amps that will be applied by the application circuit. If possible, use contacts rated higher than the volts and amps that will be applied.
  • If you need more poles than are available on one relay, you can use multiple relays with the coils connected in parallel.

Using a Multimeter Series – Resistance Measuring Basics

“Resistance is futile, Buwhahaha” exclaimed the villain, rubbing his hands together in glee as our hero struggled to free himself from the ropes that bound him.

Yep, we’ve all seen those Grade B movies.  Maybe in that situation resistance is futile, but in the world of electronics and HVAC controls resistance (electrical resistance) is very useful!

Designing selected values of resistance into the appropriate points of circuits lets us control the voltages and currents so the circuits perform as desired.  The most popular forms of temperature sensors used in HVAC (RTDs and thermistors) operate by varying their electrical resistance as temperature changes.  Resistors are also frequently used to convert the 4-20 mA signal from a transmitter into a 1-5V or 2-10V signal for controller inputs.

Electrical resistance is measured in units of Ohms.  When resistance gets into values larger than 1000 ohms it may also be expressed in Kilohms or just ‘K’ for short (50K = 50,000 ohms).  The symbol for resistance on electrical diagrams looks like this:

We’ll be measuring resistance with the Ohms function of our “multimeter” which can measure volts, current, resistance, and possibly other things too (frequency, capacitance, temperature with accessory probe, etc.).  When the multimeter is set to the Ohms function, it may be referred to as an “ohmmeter.”

Plug the Meter Leads Into the Correct Meter Jacks

Shown below is a typical multimeter face layout.  This is an actual meter Kele uses in our internal training classes.  Note that your multimeter controls may be arranged somewhat differently or completely differently:

To read resistance, plug the black meter lead into the COM jack and plug the red meter lead into the V/ohm jack.

Set the Meter Selector to Ohms

The meter selector switch has different major areas for choosing whether you want to read voltage, current, resistance (ohms), or possibly other things.  You need to move the selector switch to one of the positions in the Ohms area (left side of selector knob on our example meter).

Set Meter Range (Unless You Have an Auto-Ranging Meter)

Our example meter has different resistance ranges to choose from based on the maximum resistance you expect to measure.  Always choose the smallest range that’s higher than the highest resistance you are expecting to measure.  For example, if you are going to measure a resistance you think should be around 10K (10,000 ohms) then on the meter you would select the 20K range (because the 10K we want to check is higher than the next lower range which is 2K).

If you accidentally select a lower range than the resistance you are trying to measure, the meter won’t be damaged.  You’ll get some kind of “overrange” indication on the display.  This can vary from meter to meter.  Sometimes it’s a row of horizontal dashes, sometimes it’s “OL” for overload, or maybe something completely different.

If your meter has an Auto-Ranging function you don’t have to worry about setting the range, the meter will figure it out for you.  It will automatically step through the different ranges until it finds the lowest range that does not result in an over-range condition.

Auto-Ranging is very handy.  The down side is that it can take the meter longer to display a final stable resistance reading because it has to trail-and-error each time to find the right range.  A fixed-range meter manually set to the correct range will stabilize to a usable reading faster since it doesn’t have to experiment to find the correct range.

If the Resistor Is Connected In A Circuit, Disconnect At Least One End of the Resistor Before Taking A Measurement

If the resistor is connected in a circuit, at least one end of the resistor must be disconnected from the circuit before connecting the meter probes.  If the resistor is left connected to other devices at both ends, you will be measuring the “equivalent resistance” of other circuit components in parallel with the resistor of interest.  This will give a false reading that is lower than the actual resistor value.

It would also be a good idea to have the circuit powered down even though one end of the resistor is disconnected.  The connected end of the resistor could be attached to a live voltage or current source and bad things could happen if the probes slipped.

Place the Meter Probes Across The Resistance To Be Measured

Remember to keep your fingers on the insulated probe handles, don’t touch the metal probe tips with your fingers.  Your body is a resistor from hand-to-hand too, and if you grab both metal probe tips and try to read a high resistance value, your body resistance in parallel will introduce a measurement error.

If the display shows an over-range condition, just move the meter ohms selector to the next higher range and try again.

Special Notes On Temperature Sensor Resistance Measurements

If you try to measure the resistance of a thermistor or RTD temperature sensor with a long wire run attached, you may find that the readings jump around on the display.  This is caused by that long wire run acting as an antenna and picking up 60 Hz power line noise:

Some meters may have better noise filters than others, if you have more than one model meter available you might try them all to see which is better at noise rejection.

If the resistance value is jumping around, look for the minimum value that ever shows up on the display and the maximum value that ever shows up on the display.  Chances are that the true resistance value will be approximately halfway between those two values.

You might also notice that when you put the meter probes on the thermistor or RTD sensor, the resistance value starts to slowly move (up for RTDs, down for NTC thermistors).  It’s unlikely (although remotely possible) that you have an unstable sensor, it’s more likely you are seeing the effects of self-heating in the sensor.

The ohmmeter passes a small current through the sensor resistance in order to measure it.  This small current causes a slight heating of the sensor material.  Since temperature sensors are specifically designed to give large resistance changes with temperature change, this self-heating  shows up as a slow resistance change:

This is nothing to worry about and does not mean that the sensor is bad.  The initial sensor resistance value (before self-heating takes effect) more accurately represents the temperature of the medium being measured by the sensor.

Temperature sensors with a small thermal mass and slow-moving media are more likely to exhibit self-heating effects than sensors with a large thermal mass and swiftly moving media.  For example, a chip sensor sitting up in the air on two thin wires inside a wall-mount room housing will exhibit more self-heating effect than a duct sensor potted in the end of a metal tube placed in the moving air stream inside a duct.

Continuity Checks Using An Ohmmeter

Sometimes you simply want to know whether two points in a control panel or on a module terminal block are connected directly together.  The ohmmeter is an excellent tool for doing continuity checks.

To perform a continuity check:

  1. Make sure the circuit is powered off.
  2. Place the ohmmeter probes on the two points to be checked.
  3. If the resistance is less than 1 ohm, the two points are very likely directly connected.

Why wouldn’t the resistance read zero ohms if the points were directly connected?  Because all conductors icluding the ohmmeter test leads have some electrical resistance.  You can prove this by shorting the ohmmeter test probes directly together and observing the display, it will show a non-zero (but very small) resistance value.

Take-Away Points

  1. If the resistance to be measured is mounted in a circuit, disconnect at least one end of the resistor from the circuit before measuring .
  2. Noise pickup on long wire runs can make the resistance value jump around on the meter. Look for the lowest and highest displayed values and take the midpoint between the two as the true value. Try a different meter to see if it has better noise filtering.
  3. Slowly changing temperature sensor resistance after connecting the meter is probably self-heating of the sensor element and not a bad sensor.
  4. When taking continuity measurements with an ohmmeter, don’t expect to read 0.00 ohms across two connected points.  Any value less than 1 ohm is a good indication the points are directly connected.

Using a Multimeter Series – Voltmeter Basics

If you live in the world of HVAC design/installation, sooner or later you’re going to need to take measurements on a circuit using a voltmeter (even if it’s “not your job,” we all know how that goes).

So we thought it would be a good idea to put together some basic instructions on using a voltmeter. Even if you’ve been using a voltmeter for years, there might be some tidbit of information here that you hadn’t thought about before. :-)

Voltmeter or Multimeter?

These days you’d be hard pressed to find a test meter that just measured Volts and nothing else. Everyone manufactures multimeters which measure volts, current, resistance, and possibly other things too (frequency, capacitance, temperature with accessory probe, etc.). But today we’re just going to concentrate on making voltage measurements with our multimeter.

Plug the Meter Leads Into the Correct Meter Jacks

This seems obvious, but this author has failed to do this many times. Shown below is a typical multimeter face layout. This is an actual meter Kele uses in training classes. It’s a few years old but the functionality of the controls is the same as a modern multimeter. Note that your multimeter controls may be arranged somewhat differently or completely differently. No one likes to do it, but you might want to actually read your multimeter instruction manual if it hasn’t been thrown away/lost by now!

To read voltage, plug the black meter lead into the COM jack and plug the red meter lead into the V/ohm jack.

Do not mistakenly leave the red meter probe in the mA or Amps jack and try to measure voltages between two points in a circuit. In the mA or Amps mode the meter leads essentially look like a direct connection and you will be shorting out the two measurement points in the circuit. Bad things can happen. Since we are discussing voltage measurements today, that’s all we’re going to say on the subject. You have been warned.

Set the Meter Selector to Volts 

The meter selector switch has different major areas for choosing whether you want to read voltage, current, resistance (ohms), or possibly other things. You need to move the selector switch to one of the positions in the Volts area (upper right area on our example meter).

Set Meter Range (Unless You Have an Auto-Ranging Meter)

Our example meter has different voltage ranges to choose from based on the maximum voltage you expect to measure. Always choose the smallest range that’s higher than the highest voltage you are expecting to measure. For example, if you are going to measure 24V then on our meter you would select the 200V range (because the 24V we want to check is higher than the next lower range which is 20V).

If you accidentally select a lower range than the voltage you are trying to measure, the meter won’t be damaged, you’ll get some kind of “overrange” indication on the display. This can vary from meter to meter. Sometimes it’s a row of horizontal dashes, sometimes it’s “OL” for overload, or maybe something completely different.

The only way you should damage your voltmeter from overvoltage would be if you exceeded the maximum rating for the meter. That value should be in the instruction manual and it’s almost always printed on the meter face too. It’s pretty high, typically something like 750V or 1000V, values you will probably never encounter in HVAC work.

If your meter has an Auto-Ranging function you don’t have to worry about setting the range, the meter will figure it out for you. It usually starts on the most sensitive range and, if it sees an over-range condition, moves to the next higher range etc. until it finds the most sensitive range that does not result in an over-range condition.

Auto-Ranging is very handy. The down side is that it can take the meter longer to display a final stable voltage reading because it has to trail-and-error each time to find the right range. A fixed-range meter manually set to the correct range will stabilize to a usable reading faster since it doesn’t have to experiment to find the correct range.

Set Meter for AC or DC Volts As Needed

 If you set the meter to read DC volts and put the probes on a AC voltage source, the meter will read essentially zero volts (the readout might jump around the zero reading a bit).

If you set the meter to read AC volts and put the probes on a DC voltage source, the meter display will jump up momentarily then “coast” back down to essentially zero volts over time.

So be careful – an incorrectly set meter will make you think there is no voltage present when there really is.

If you are probing a “mystery circuit” and you’re not sure whether the voltage between two points is AC or DC, you can try both settings on the meter to see which gives you a non-zero value.

Place the Meter Probes on the Circuit Points To Be Measured

To make a voltage measurement, you do not need to disconnect anything in the circuit. You do that for current or resistance measurements (which we are not covering today).

Remember to keep your fingers on the insulated probe handles, don’t touch the metal probe tips with your fingers. You might know the voltage is supposed to be low (24V) but why take a chance in case you’re mistaken or there’s a short in the wiring?

In the case of AC voltage there is no polarity to worry about, the signal will always read positive on the display no matter which way the probes are placed.

In the case of DC voltage, the red probe should go on the more positive point and the black probe should go on the more negative point. But if you should get it backwards no harm is done, the meter will just display a negative voltage value, the magnitude of the reading will still be correct and you will know that the point with the black probe is actually the more positive point.

If the display shows an over-range condition, just move the meter voltage selector to the next higher range and try again.

Average-Reading Versus True-RMS AC voltmeters

Not all voltmeters are created equal when it comes to measuring AC volts. There are two different measurement techniques in use.

“Average-Reading” AC voltmeters only give a correct reading if the AC voltage is a sine wave. Most AC voltage signals we read in the HVAC world are sine waves (or very close) so this is typically acceptable. Less expensive meters tend to use the average-reading measurement technique.

“True-RMS” AC voltmeters will give a correct reading no matter what wave shape (does not have to be a sine wave). This technique is typically reserved for the more expensive meters.

Do Newbie Practice On Low Voltages

If you’re new to taking voltage measurements with a voltmeter, we recommend you start with a low-voltage source just to keep things really safe. A step-down transformer with a 24VAC secondary or a bench power supply with a low voltage DC output would be great.

Follow the recommendations above and taking voltage measurements with a multimeter should be second nature in no time at all!

“Currently” Playing: Connecting A 4-20 mA Constant-Current Signal To Multiple Loads

One of the most popular and long-lived methods for transmitting analog control signals in the HVAC and industrial control worlds is the “constant-current signal loop.” In this scheme, the value of the current flowing in the circuit is the control variable, rather than any voltages that may appear at different points in the circuit. The constant-current loop is very robust and noise-tolerant and can run for long distances. Even in this high-tech digital age, it remains a very popular analog signaling method.

This article will deal with how to connect multiple loads to a constant-current signal loop. Connecting multiple loads to a constant-current signal loop is totally different than connecting multiple loads to a voltage signal source (which is covered in a previously-published article for those interested).

In the HVAC world, the most common range of current used for constant-current signaling is 4-20 mA (1 mA = 1 milliampere = 0.001 amperes of current). 4 mA represents 0% of the variable’s value and 20 mA represents 100% of the variable’s value. Other current ranges can be used, but today we’ll work exclusively with 4-20 mA current loops.

Current loop load “resistance” or “impedance”

Every load to be connected in the current loop will have an electrical characteristic called “resistance” or “impedance.” The load’s datasheet might use either term. Don’t worry about whether the datasheet says “resistance” or “impedance” we’ll take them to be the same thing. In this article we’ll use the term “impedance.”

Impedance is measured in units called “ohms.” If you don’t know what ohms are, don’t worry. All you have to do is use the numbers in some simple calculations later on.

Basic constant current loop configuration

Before we discuss connecting multiple loads to a constant current loop, let’s get familiar with the basic constant current loop containing a single load:

currently figure 1

On the left we have a constant-current source that is regulating the current to some desired value (the desired value is determined by a calculation in a controller, measured physical value in a sensor, etc.). The constant current leaves the source, flows through the load, and returns to the source.

Simple, right? What could go wrong? As usual, “the devil is in the details.”

Two different styles of current sources

There are two different styles of current sources you should be aware of. We’ll call the first style the “locally powered” current source. It has its own source of power for the 4-20 mA output and the only other item needed to complete the loop is the load:

currently figure 2

You will usually find this type of current source on the output of a powered controller.

We’ll call the second style the “loop powered” or “2-wire” current source. It does not have its own power source built-in, but rather depends on a separate power source (usually 24VDC) wired into the loop:

currently figure 3

You will find this type of current source in many 4-20 mA loop-powered temperature, pressure, and humidity transmitters.

Current sources have a “maximum load impedance” they can drive

Current sources like small values of load impedance, in fact they can drive their current straight into a short circuit (zero ohms load impedance)!

Current sources run into trouble when the load impedance gets too large. When the load impedance gets too large, the current source will not be able to make the higher values of current (near the 20 mA end) flow. The current value will plateau at some value less than the desired value.

How do I know the “maximum load impedance” my current source will support?

For a locally-powered current source, you will need to find the maximum load impedance value on the product’s datasheet.

For a loop-powered current source, you may find a parameter called “maximum load impedance (with 24V supply)” on the datasheet. If you do not, you will have to perform the following steps:

1. Find a parameter called “minimum operating voltage” (or some very similar wording) on the current source datasheet.

2. Calculate maximum load impedance as follows:

Maximum load impedance = (Loop power supply – minimum operating voltage) / 0.020

For example, suppose we are using a 24V loop power supply and we see on the datasheet that the 2-wire current source needs a minimum of 11V to operate. Then we would calculate:

Maximum load impedance = (24V – 11V) / 0.020 = 650 ohms

Now that we know something about how a single-load constant-current loop works, let’s see what kind of complications we get into when trying to drive multiple loads!

Connecting multiple loads to the current loop

Rule #1: All loads must be connected in series around the loop, never in parallel.

Below are sketches of the right way (series) and the wrong way (parallel) to connect loads in a current loop:

currently figure 4

currently figure 5

At first glance this requirement seems simple enough. That’s because we’ve drawn all the loads as 2-wire loads that don’t require power supplies. A not-so-obvious problem may occur if the loads do require power supplies. The current will always take the “path of least resistance” back to the current source whatever that path might be. If one of the loads before the last load has a power connection back to the current source, this connection will be the “path of least resistance” and the 4-20 mA will bypass the rest of the loads in the loop:

currently figure 6

If the powered load is a standalone module, you may be able to use a dedicated “floating” transformer or small DC power supply to power the load and resolve the issue:

currently figure 7


If multiple loads in the series connection are powered, each load would need its own power supply that is floating with respect to all the other supplies. For example, if Load 1 and Load 2 were both powered, Load 1 would need a power supply that is floating with respect to Load 2 and the current source, and Load 2 would need a power supply that is floating with respect to Load 1 and the current source. It can get messy quickly if there are several powered loads in the loop!

Consider also that If the powered load is a controller input and the controller is serving other inputs and outputs, it wouldn’t be practical to “float” the entire controller subsystem’s power connections.

In cases where a floating power supply for each load is not practical, it’s time to employ a signal isolator module such as the DT-13E from Kele:

currently figure 8


The DT-13E signal isolator has its inputs isolated from its outputs, and both inputs and outputs are isolated from the power terminals. So with the DT-13E installed, it just doesn’t matter where Load 2’s power source comes from, it will not corrupt the 4-20 mA current in the loop.

Rule #2: The sum of all the load impedances must be less than the maximum load Impedance the current source can support.

When the mA loads are wired in series (as they must be) the total load impedance seen by the current source is the sum of the individual load impedances. So if each of the three loads in the drawing below is 250 ohms, the total load impedance seen by the current source would be 250 + 250 + 250 = 750 ohms:

currently figure 9


Suppose your current source datasheet shows that it can only drive 650 ohms maximum impedance? Houston, we have a problem…

What can you do if your total load impedance adds up to more ohms than the current source can handle? This is another way that the DT-13E signal isolator can sometimes help. Many HVAC 4-20 mA loads have impedances of 250 to 500 ohms, but the DT-13E mA input impedance is only 125 ohms. So by isolating one or more loads using DT-13Es, not only do you remove grounding concerns but you may also lower your total loop impedance to a value the current source can handle:

currently figure 10


Now the current source only sees 250+125+250 = 625 ohms which is lower than its maximum load impedance, and Rule #2 is satisfied.


Multiple loads inserted in a 4-20 mA current loop must be connected in series. If the series-connected loads are powered devices, “sneak paths” between the load power supply connections and the current source may misdirect the 4-20 mA so that not all the loads receive the signal. Isolating the series-connected load with a DT-13E or similar signal isolator will solve this problem.

The sum of the series-connected load impedances may be too high for the current source to handle. The DT-13E signal isolator’s impedance of 125 ohms is less than that of many common HVAC loads. Isolating a load with the DT-13E may lower the total loop impedance to an acceptable value.

Networking 101: BAS Network Cabling

The various devices comprising a building automation network are either connected by cables or talk wirelessly to each other, or possibly a combination of both.  In Networking 101, we are going to concentrate on network devices connected by cables.  Let’s take a look at the different types of network cables typically encountered in building automation networks.

Network Cables 

The most common type of network cable used in building automation networks is the “twisted pair” cable.  Two wires are twisted around and around each other at a uniform number of twists per inch to form the “twisted pair”: Screen Shot 2014-09-24 at 2.19.53 PM Why twisted pair cable?  Without going into a lot of technical detail, let’s just say that with the proper type of transmitters and receivers using twisted pair cable greatly reduces noise pickup from noise sources such as motors, fluorescent lights, and radio stations.  This results in more reliable communications with less corrupted data and retransmissions.

Not all twisted pair cables are created equal.  There are electrical characteristics such as resistance and capacitance per foot that vary between different brands and models of twisted pair cable.  Generally the lower the resistance and capacitance of twisted pair cable, the better it performs in networks.

It’s a good idea to select a network cable that’s specifically recommended by the manufacturer for use in your type of network.  If network-specific cable is not available, substitutions can be made.  If the resistance and capacitance per foot of the proposed substitute cable is as low as or lower than the recommended cable, the substitute cable should be acceptable.

Some styles of network cable are available with or without a metallic shield around the outside of the wires.  Shielded cable generally helps with reducing noise pickup, but the cable will cost more.

Now let’s get more specific about the types of physical layers typically found in building automation networks…


RS-485 is used extensively in building automation networks.  Modbus RTU, JCI Metasys N2, and BACnet MSTP protocols all use RS-485.

RS-485 uses a single twisted-pair to send the data.  It also uses a third “reference wire” in addition to the twisted pair.  There is debate among users as to whether the reference wire is always needed, and some devices do not include a terminal for connecting a reference wire.  But to be safe it’s always best to include it in the cable: Screen Shot 2014-09-24 at 2.22.13 PM RS-485 devices typically use screw terminals for connecting to the cable.

The two wires of the twisted-pair do have a polarity assigned to them.  The same wire of the pair must be connected to the RS-485 “+” terminal of all the devices on the network.  If an RS-485 device is connected to the network with the “+” and “-“ wires attached backwards, it will not talk on the network!

RS-485 cable should be run as one continuous “trunk line.”  The RS-485 standard specifies that as many as 32 devices can share the same trunk line, and the line can be as long as 4000 feet.

It is bad practice to “tee” into the middle of an RS-485 cable and run a “stub” line off to another RS-485 device:


Instead, the RS-485 line should be daisy-chained to each device on the line like this: 4

If an RS-485 cable needs to be longer than 4000 feet a “repeater” module can be inserted in the middle of the cable run.  The repeater will refresh the electrical signals and allow for longer runs.

You can also expand an RS-485 network to more than 32 devices by creating multiple sub-networks, each with its own cable, and connecting the sub-networks together with a “bridge” or “router” device.  A bridge or router will have two or more separate RS-485 electrical ports.  Each port on the bridge/router connects into one of the sub-networks and data can then flow between the sub-networks:

5 A “Bridge” forwards all the traffic on either sub-network across to the other sub-network.

A “Router” has the capability to learn (or to be told through a user-generated lookup table) which devices reside on each sub-network.  Now if two devices on the same sub-network are talking, the conversation is not passed to the other sub-network.  This eases overall network traffic congestion.

FT-10 “Free Topology” (Lonworks)  

The FT-10 “Free Topology” physical layer was invented by Echelon Corporation as part of their Lonworks networking technology.

Similar to RS-485, FT-10 uses a single twisted pair to carry the data, but FT-10 does not use a third  “reference wire”:

6 The FT-10 twisted pair has no polarity – the wires can be attached to each Lonworks device without worrying about which wire is “+” and which is “-“.

Like RS-485 devices, Lonworks FT-10 devices also typically use screw terminals to connect to the network cable.

FT-10 is much more relaxed as to how the cables can be arranged – you can pretty much wire different FT-10 network segments together any way you want to.  You can “tee” into an FT-10 line with a “stub” headed in another direction, or you can make a “star” with a center point and multiple lines radiating out from it:

7 Lonworks FT-10 documentation recommends that the total cable run be no longer than 1640 feet and you put no more than 64 devices on a network.

As was the case with RS-485, you can extend the FT-10 cable distance with an FT-10 repeater.  You can also extend beyond 64 devices by creating sub-networks and connecting them together with an FT-10 bridge or router.

Please note that even though RS-485 and FT-10 both use a single twisted pair to carry the data signals, they are totally incompatible electrically!  You cannot ever tie an RS-485 network cable directly to an FT-10 network cable.  

If you want to communicate between an RS-485 network and an FT-10 network, you must use a “Gateway” device which has an RS-485 port on one side and an FT-10 port on the other side:

8 Ethernet  

Ethernet was invented back in the 1970s by Xerox and is extremely popular in the Information Technology world.  BACnet IP and Modbus TCP protocols use the Ethernet physical layer.

There are several different versions of “Ethernet” networks, but the two we deal with in building automation are the 10BASE-T and 100BASE-T versions.  The only significant difference is the speeds.  10BASE-T runs at 10M (10 million bits per second).  100BASE-T runs at 100M (100 million bits per second).

10BASE-T and 100BASE-T devices can be mixed on a network.  If a 100BASE-T device needs to talk to a 10BASE-T device, it simply drops its speed to the 10BASE-T rate temporarily.

The two styles of Ethernet cables you’re likely to encounter in building automation are designated as CAT5 (Category 5) and CAT5e (Category 5 Enhanced).  Both cable styles can be used with either 10BASE-T or 100BASE-T, but CAT5e cable gives superior performance.

The 10/100BASE-T cable contains four twisted pairs, but only two of them are used to carry the data: 9 The other two twisted pairs may be left unused or used for some other purpose.

Standard length off-the-shelf 10/100BASE-T cables typically come with RJ-45 plugs pre-installed on the ends of the cable.  RJ-45 plugs look like modular telephone plugs except they are wider and have positions for 8 connections.  If you need to make a custom length 10/100BASE-T cable, you can buy a bag of loose RJ-45 plugs and a special wire stripper/crimper tool and attach RJ-45 plugs yourself (but it is a bit tedious to do).

Unlike RS-485 and FT-10, you can only connect two devices with a 10/100BASE-T cable!  And the recommended maximum cable length is only 330 feet.  So how are we able to network many devices together?  The answer is in the use of Ethernet cable-sharing devices such as “hubs” and “switches.”

These hubs and switches have multiple RJ-45 data jacks for plugging in Ethernet cables.  They are used to build out an Ethernet network “like a tree”:

10 An Ethernet ‘Hub” forwards everything it receives on any port out all the other ports on the hub.  So if an Ethernet network is built using all hubs, every end device on the network must listen to every conversation on the network whether it’s involved or not.  This creates a lot of unnecessary traffic and slows down the operation of the network.

An Ethernet “Switch” learns where the different end-devices are located in the network and restricts the traffic flow to only those network segments that are needed to reach the specified end device.  This greatly reduces unnecessary traffic on the network and speeds up response time.

An Ethernet “Router” is used to connect multiple Local Area Networks (LANs) together to form a Wide Area Network (WAN).  There are also Ethernet “Gateways” to connect Ethernet networks to RS-485 or FT-10 networks.


RS-232 is a physical communications layer that is used for making point-to-point connections between 2 pieces of equipment.

You may find that some network devices include an RS-232 port for connecting to a computer to perform device configuration.  This RS-232 port is not for the network connection, only for configuring the device prior to using it on a network.

RS-232 ports on older equipment used 25-pin connectors like this: 11 Modern equipment providing RS-232 ports use 9-pin connectors like this: 12 The connectors come in both male and female versions, so when connecting two pieces of equipment you must the sure that the cable has the correct gender connectors.

A “normal” RS-232 cable will have a male connector on one end and a female connector on the other end with “straight-through” wiring (pin 1 connects to pin 1, pin 2 connects to pin 2, etc.).  But sometimes a custom cable must be purchased or fabricated with male-male or female-female connectors and possibly with crossed wiring between the pins.  The details of these special RS-232 cables are beyond Networking 101.

Q: The intrinsic safety spec I’m reading calls for an isolated ground. Isolated from what?

Answer: A true isolated ground is not connected to any ground that can ever carry fault current from unrelated parts of the electrical system. It is best to run it directly to grounded building structural steel, an underground metal water pipe, or a separate grounding electrode from the building electrical service as described in Article 250 of the National Electrical Code. However, many grounds that claim to be “isolated” are actually just separate wires run back to the ground bar on the nearest panelboard.  At best, they are run all the way back to the service entrance ground. In either of these cases, a high-current ground fault in the electrical system can raise the potential of the ground wire to destructive levels. True isolation is important for sensitive electronic devices, and is especially important in intrinsically safe systems where an explosion could result from a high voltage appearing on a ground conductor.