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Newbie here.

We have a 3 phase installation for our dimming control system for our lighting system. During inspection while our 3 phase is safe with lock out tag out. Normally, we will get a low voltage reading from phase to phase less than 3 V.

This time, we encountered a phase to phase low voltage reading more than 3 V with a fluctuation up to 6 V.

  • What can cause this higher voltage?
  • What is this called, Unbalance load?
  • How can we correct this?
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  • \$\begingroup\$ Phase to phase is line voltage so, how can it be 3 volts? \$\endgroup\$ – Andy aka Feb 29 at 10:55
  • \$\begingroup\$ Hi Andy, Our phase to phase reading e.g L1-L2 ON state is 397 volts. But during OFF state we're getting reading 3 volts and higher in one of our dimming station. Our other dimming station has voltage less than 2 volts. Is this normal? \$\endgroup\$ – Marlon Feb 29 at 11:06
  • \$\begingroup\$ OFF-state? is there a mechanical switch? Or some dimmer circuit which can well leak for ex capacitively to generate some voltage that 10MOhm multimeter doesn't pull to its knees and which cannot cause any current in loads which need high voltage for any current more than zero. \$\endgroup\$ – user287001 Feb 29 at 11:49
  • \$\begingroup\$ When the Transformer 3 phase power is ON with or MCB switch it has phase to phase 240 volts reading in our lighting dimming system. But when the power is OFF we still get a low voltage reading of 6 volts phase to phase in one of our dimming control system. I know its not normal and I want to know what caused it and how to correct it? \$\endgroup\$ – Marlon Feb 29 at 12:07
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I suspect that you are measuring noise or induce voltage on the cable while using a digital multimeter with a very high (1 to 10 MΩ) input impedance.

enter image description here

Figure 1. A Fluke 117 meter with a LO-Z measurement range.

Interestingly, Fluke have a range of meters with a Lo-Z range to avoid this problem on DC and AC circuits. The input impedance is about 38kΩ (from memory). This is low enough to give a zero reading when measuring voltage on an open circuit even in the presence of 'noise' on adjacent lines.

In the absence of one of those you could connect a small light bulb or relay coil across the terminals. This will probably load the circuit enough to cause the voltage to collapse.

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  • \$\begingroup\$ noise or induce voltage? How come the low voltage reading happens only in one particular dimming system? We have 30 dimmer racks and mostly have 0-2v reading only phase to phase when there's no incoming power from the source transformer. I don't think the issue is from the testing equipment. \$\endgroup\$ – Marlon Feb 29 at 12:31
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    \$\begingroup\$ It's easy to test as explained in my answer. Obviously I can't comment on the differences between your systems based on the information provided. \$\endgroup\$ – Transistor Feb 29 at 12:52
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Capacitive (and possibly inductive) pickup between different cables in shared containment is most likely.

With the isolation open you have supply wiring that is open circuit at the isolator but that is run in the same conduit as circuits that are live (and, phase angle dimming being what it is, also possibly having high dv/dt), some capacitive pickup is to expected.

There is sometimes something to be said for an old school 20kohm/V meter for measuring this stuff.

If your 'isolation' does not cut the neutral and you are measuring to ground, then it is just barely possible that you are seeing evidence of a neutral issue, but isolation should really be ISOLATING.

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  • \$\begingroup\$ 20 kΩ/V at 10 V gives a meter impedance of 200 kΩ which would be 1/50 times that of a typical digital meter so that might do the trick. Note that the Fluke approach (in my answer) is even lower. \$\endgroup\$ – Transistor Feb 29 at 14:37
  • \$\begingroup\$ thank you for the info. \$\endgroup\$ – Marlon Mar 1 at 5:52
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As was said it is almost always not a real working voltage but do not take it for granted. One of my coworkers did and lost a thumb and forefinger due to it being a low resistance to Ground Return from a 100 HP Motor that was physically connected to the Frame Ground of the Power Gear. In most instances it is ambient floating phantom induced voltage. That said Do Not just dismiss it. It can and will on occasion hurt you! So always check it out to be sure, it is only Good Engineering and Safety Practice to make sure and protect your self. Use the light bulb load trick it will most assuredly dissipate a phantom voltage. I use a 120 volt 60 watt INCANDESCENT (NOT L E D!) Bulb they have roughly 70 ohms resistance. (Yes I know this does not work out under Ohms law, BUT that's the cold filament resistance. It just is what it is!) I just screw one into one of the old rubber 2 wire fixtures and have 36 inch clip leads coming from it so it can be hung back out of the way. When clipped across a phase the phantom voltage should disappear. As far as 3 or 6 volts that all depends upon where it is coming from initially, and how long physically the wires are laying next to each other, including if they are in a wireway with 440 wires. Also you mention dimmers they often use "Wild Voltage" which has both strange Voltages and Frequencies! Do Not assume 120 is 120 or 240 is 240 let alone try to guess the Frequency or the Wave-Shape of the power in a dimmer circuit. Wild Voltage is called that for a Reason!

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  • \$\begingroup\$ thank you for the info. \$\endgroup\$ – Marlon Mar 1 at 5:51

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