Is it a backfed or induced voltage?
Jim White, Training Director, Shermco Industries, Inc.
You're about to test for the absence of voltage and have gone through the Lockout/Tagout process. You're wearing the proper personal protective equipment (PPE), you have the right voltage tester and you know how to use it. As you touch the test probes onto the circuit you get a voltage where there should be none! What's going on?
1. You might be on the wrong piece of equipment. Certainly not you! This is a big enough problem that the NFPA 70E included a new article about it in the 2009 edition in Article 130.7(E), Alerting Techniques, "(4) Look-Alike Equipment. Where work performed on equipment that is deenergized and placed in an electrically safe condition exists in a work area with other energized equipment that is similar in size, shape, and construction one of the altering methods in 130.7(E)(1), (2) or (3) shall be employed to prevent the employee from entering look-alike equipment."
2. Is it really off? If the breaker or fuse feeding our circuit is not clearly marked or if a molded-case circuit breaker has tripped, shocking things happen! I have been bit more than once from a "tripped" molded-case circuit breaker only to find out the contacts did not completely open. They wouldn't carry current, but they sure carried voltage! Always set a tripped breaker to the full OFF position before working on it.
3. Induced or "ghost" voltages could be present. Many people think induced voltages only happen in outdoor, high-voltage substations. While that's the biggest danger from induced voltages, low-voltage circuits run in cable trays can also induce a voltage into deenergized cables that are in the same cable tray (see Figure 1).
Figure 1 Low-voltage Induced voltage scenario
Applying a static ground to this circuit would dissipate the voltage without problem, since an induced voltage does not have any short circuit current capability.
4. It could be backfed. Control power transformers (CPT), indicating lights and "foreign" circuits (those coming from another panel or area) can be the culprit. Applying a static ground to a backfed circuit could cause arcing, which is unsafe.
Often, backfed voltages and induced voltages can be very similar. Induced voltages are typically much lower than the circuit's nominal voltage, but backfeeds can be in the same voltage range as induced voltages. Since it is not safe to ground a backfeed, what can we do?
Backfed voltages are voltages that often originate from another circuit or part of the equipment, but "backfeed" through indicating lights, control power transformers or even resistors in equipment. These voltages are usually less than the nominal voltage of the circuit and can be approximately the same value as induced voltages. It can be difficult to tell the difference between a backfeed or induced voltage. If an induced voltage is connected to ground, there is no generation (current) source and the voltage will dissipate. A backfed voltage, even though it is lower than nominal, does have a generation source feeding it and will arc when connected to ground.
Low vs. high impedance test tools
The solution is to use a combination of test tools in order to determine whether it is backfed or induced, and then verify the initial results.
Good-quality voltage testers typically have a high-input impedance. I found out the value of this when I was testing a 9,000 ton chiller that had an intermittent problem. I connected the test probe to one side of the coil and when I touched the other probe to ground, the coil closed, tripping the chiller off-line. This was not a career-enhancing moment. The meter I was using had an input impedance of only a few thousand ohms. When I made the connection between the energized coil and ground, enough current flowed through the meter to operate the coil. A meter with a high-input impedance would not allow enough current through the meter to cause the coil to operate. I took my inexpensive, low-input impedance multi-meter home and bought a good quality unit, one that had a high-input impedance.
So, after first measuring voltage with a standard, high-input impedance voltmeter, use a digital multimeter with a low-impedance option, such as the Fluke 117 or 289. These meters offer both a high-input and a low-input impedance function. If the voltage is induced, the low-impedance input should dissipate the voltage once it is connected to ground. Using a low-voltage proximity tester, measure along the circuit being tested while the low-impedance voltage tester is still connected. Figure 2 illustrates the end readings; no voltage indicated by the proximity tester and no voltage displayed on the low-input impedance tester.
Figure 2 Voltage backfeed indications
If the low-input impedance voltage tester does measure a voltage, as in Figure 3, even though it may only be several volts and the proximity tester indicates the presence of voltage, the voltage on the circuit is probably a backfeed and needs to be found before proceeding. Applying a ground on this circuit would result in arc welding!
Figure 3 Backfed voltage indication
A dual-impedance meter is perfect for this test - better than carrying two separate meters or making an unsafe measurement.
If you find a circuit that shows voltage when there should be none, be careful what you do next. Creating an arc is unsafe and could get you fired or much worse. Be safe. Determine whether the voltage is induced by nearby, energized cables or if it is being backfed from an unknown source.
Read part one of this Electrical Testing Safety series: Preparing for the absence of voltage testing