DMM read 142.2 V on nom 120 V source w/ poor conn. 1

[trihydrate]

I worked on a 120 VAC system that had a water conductivity instrument with a 125 mA time delay fuse that had been blown. I first replaced the instrument and after 30 hours, the second instrument's fuse had blown. A 250 mA fuse was installed and it blew immediately. I placed a DMM on the power lines to the instrument, set it for voltage and selected "Max Hold". For a few minutes, the reading climbed slowly from 122.2 V to 122.8 V. As some other equipment, a solenoid, some lights etc. started operating, the DMM suddenly went to 132.1 V. After a few seconds, the DMM went to 142.2 V.
Suspecting a shorting to another phase, I reviewed the supply wiring, and confirmed their was no shorting. What I found was the 20 amp circuit breaker in the distribution panel had been forceably installed with the spring clamp jammed beside the bus bar rather than over the bus bar. The system had been operating for six months and had finally through many on-off current cycles, started weakening the elecrical contact and produced the problems identified.
Replacing the circuit breaker stabilized the voltage readings and a new instrument is working fine.
Question #1 is "What caused the fuses to blow(and in the case of the second instrument, damage the circuitry)?" and question #2 is "How did the DMM see a voltage reading of 142.2 V with a poor connection to a nominal 120 V source?.

Thanks for your help!

 

[jOmega]

Have seen this before, but not in a long time.

Take a look at how the 120v is routed to the instrument.
Is it bundled with power wires conducting 230v or 460v ?

If so, what you may be experiencing is an induction voltage. When current passes thru a wire, a flux field is established around the wire. Adjacent wires can be influenced by this field thru transformer action.

I think an important clue here is that as load picked up the voltage climbed. As the load picked up, the field around the wire carrying the load got stronger.

Solution was to separate the wires... 120v wires run together, 230v wires run together... and 460v wires run together.

Another trick is to use a twisted pair (5-8 twists/inch) for the 120v wiring to the instrument.

Looking at the 120v with an oscilloscope is highly recommended. Use an ungrounded scope... and becareful not to touch the scope case and ground at the same time. Could be deadly.

Good luck

 

[electricpete]

You mentioned the voltage increased when other loads came on.

That triggers one thought which would be unrelated to circuit breaker condition.

If there is an open circuit in the neutral connection of your service, then you have one leg with "+"120v, one leg with "-"120v (ie 180 degrees out of phase), and the neutral leg floating somewhere in between. Where the neutral goes between -120v and +120v depends upon what loads are on each circuit.

Of course in that situation you would rarely expect to measure 120v to neutral.

 

[trihydrate]

jOmega and electricpete,

Thanks for your comments. I need to clarify a couple of points. First is, I have been told and believe there has been no wiring changes in the last six months. This would tell me if induction voltages were the problem, it would have been a problem for the last six months. There is 230 V, 3 phase in the same conduit. My experience with induction voltage is in an unpowered conductor, the induced voltage is certainly present, but the available current is very low, on the order of a few mA. Also once the conductor becomes powered, the induced voltage is not detectable.

The problem I had in another case, was of a small relay being actuated by the powered conductor and then when the power was removed, the induced volage/current from other wiring exceeded the holding current required to keep the relay actuated.

The second point is the DMM was displaying the highest voltage reading obtained since engaging the "MAX HOLD" switch. By releasing the "MAX HOLD" switch the DMM always went back to the 122 V range. I believe what the DMM saw and held, was spikes or noise. If that is true, what mechanism exists for the poor connection to produce even a miniscule increase in voltage, or is the detection circuit of the DMM giving a false reading, or should I just take two aspirin and forget the whole thing?

Thank again!

 

[busbar]

Offhand, all bets may be off trying to interpret a transient/damped-oscillatory signal on a digital-multimeter AC scale, particularly if it is initiated by a series arc.

 

[ve7brz]

I'm wondering if your poor conection was beginning to operate as a diode, prohibiting the back EMF from a solonoid from being absorbed back into the supply side of the system.