My customer wants me to monitor DC current from his battery room to his UPS (400A full load). I'm stuck - current transformers don't read DC, do they?

Answer:

You're right - the current transformers (CT's) in stock at Kele are not designed for DC primary currents, but we do have access to DC CT's and have provided them for similar applications. Unfortunately, they are quite expensive, so here's an alternative. A conductor carrying a DC current is generally sized according to the minimum that the NEC allows. If this is the case, there will be a measurable voltage drop along the length of the conductor. It will typically be in the range of 0.1 V to 0.4 V per 10 feet of length at rated load. Since the conductor is in fact a resistor, this voltage drop will vary in a linear fashion with the current level. (Table 8 in Chapter 9 of the NEC is a good source for estimating conductor resistance.) If you calculate or measure the voltage drop at maximum current flow, a Kele Model UAT Universal Analog Transducer can be set up to convert this drop to a 4-20 mA, 0-10 V, or another signal proportional to DC current. You need only to connect voltage-sensing leads to each end of the positive conductor, and fuse them according to their ampacity.

Don't be tempted to use the battery system for control power, even if it happens to be 24 VDC. The UAT power and signal share a common terminal. Power supply connections to the UAT must be ungrounded and totally isolated from the battery system, or else a short circuit of the high current battery bank can result. In addition, a DT-13 Signal Isolator is required if the UAT's 4-20 mA output is connected to a grounded (non-floating) input.