Perplexed!

Status
Not open for further replies.
P

pierre

Senior Member
Location
Westchester County, New York
Here is the situation:

A new installation of a generator. the generator is installed on the 34th floor (roof) of a large building. There are control conductors in a conduit run down to the basement - 34 floors- with 8 junction boxes between. The control conductors are 14 THHN with a 1-2 amp load. The conductors are run from the fuel location in the basement to the control panel inside the housing of the generator on the roof.

They ran 6 conductors for the controls, and 14 spare conductors. The spares conductors were not terminated at either end.
In the junction boxes, there is a metallic barrier the same gauge as the box. One side has the control conductors, the other side has the 480v conductors from the generator to the load locations.

Without any load connected/or the generator actually running we have an unusual situation.
NOTE: the control conductors are for alarms to notify when the fuel source is low, or out - to shut down the generator.

when the control circuit is tested, all the unused (spare) conductors develop 54 volts. This is creating havoc with the alarms.
If a spare conductor is bonded to the enclosure at one end, 2 volts are shaved off of the 54 volts. Bonding the other end of the same conductor shaves off another 2 volts. Continuing to do this with the other spare conductors reduces the "Phathom" voltage until the alarms are not affected anymore. This is a temporary solution, until we can find the reason this is occurring.

Remember the 480v wiring has not been energized yet.


What could be the reason this is occurring?

Has anybody run across this before?

Thanks for the help, I appreciate any answer, as it may lead to solving this situation - even the generator techs have not come up with a solution, as they are protecting themselves and are saying this is a wiring mistake. Maybe they are correct, I am not sure.
 
Re: Perplexed!

Sounds like you need a shielded cable for the alarms.

Is the voltage the same at both the genset and the tank alarms?
 
Re: Perplexed!

These kind of things are always wierd. I have no idea other than it's induceds voltage, maybe. But out of curiousity I divided 54 (measured voltage) by 14 (number of conductors) and get 3.86. That's kind of close to 2 volts per conductor end. I'm not trying to say that means anything though.
 
Re: Perplexed!

Pierre, it must be inductive voltage from Something!?
What type of conduit?
Are they running parrallel to other conduits or cables?
If so, how close?
Or are they sharing a conduit with other feeds/ wires?

On another note...MAN #14 sounds awfully small for travelling that far! What voltage will be used in these wires?

Dave
 
Re: Perplexed!

Originally posted by physis2:
2 amps 350 feet. Should ought a be fine.
Click to expand...
Depends on what voltage they are using.

Pierre, check to see if the building steel and/or conduit is also energized.
just a thought.

Dave
 
Re: Perplexed!

I assume the conduit is metalic? Is there voltage on those metal jboxes which would mean on the ground of the 480V circuit?
 
Re: Perplexed!

Sounds like induced voltage. Is everyone sure this is actually the cause of the havoc or is it just assumed this is indeed the cause? Is it being measured with a high impedance digital meter ar a low impedance analog meter?
 
Re: Perplexed!

Oops, I missed the part where you said eliminating the voltage cures the problem. Nevermind, still sounds like induced voltage though.
 
Re: Perplexed!

I think I agree with Bruce. If the spare wires aren't attached to anything, how can they affect the alarms. I think you have another problem, and the phantom voltage is throwing you off track.

Note that the 54 volts is about half of 120 volts. I wonder if there is some type of 120V control transformer with a grounded center tap.

Steve
 
Re: Perplexed!

I am curious as to how you are measuring the voltage drop across a conductor that spans 34 floors. How do you attach a probe from your meter at each end of the wire?

But to answer your question, I think that there must be a strong field in the air surrounding the building. Perhaps there is a radio or cellular tower nearby. A conductive material in the presence of a varying magnetic field will experience a separation of the electrons from their home atoms: electrons go one way, and leave positively charged atoms behind. That is the voltage drop you are measuring.

I suspect that Paul?s solution would be the way to go: use shielded twisted pairs for the control cable. But this is outside my area of expertise. You need to bring in an expert on electromagnetic compatibility (EMC). There are companies that specialize in this, but I don?t know how to find one.

Perhaps a member of the Forum might be employed in this specialty, and could send you a Private Message. We don?t allow advertisement or solicitations for business, but the Forum Rules do not prohibit someone from contacting you privately.
 
Re: Perplexed!

Install relays and the coils will disipate the induced voltage and run the alarm circuits thru the dry contacts...
 
Re: Perplexed!

I'm not a signal expert either.

If the conductors are in a conduit aren't they already sheilded?

My experience with sheilding is that it isn't very effective.

I'm curious what happens if you connect them all together at either end. That could be a satisfactory solution for spare conductors.
 
Re: Perplexed!

Originally posted by hochwire:
Install relays and the coils will disipate the induced voltage and run the alarm circuits thru the dry contacts...
Click to expand...
I like this idea, too. If you go this route, you may want to install a cap across the coil.
 
Re: Perplexed!

I had a similar problem with an underground fire alarm conduit. The voltage was (we assumed) induced from another underground (line voltage) conduit. We tied all of the spare wires together at each end and bonded them to the conduit system to bleed off the induced voltage. Never had another problem! That was PVC conduit though, your probably steel and quite long. Try bonding them at each end and at all JB's (I assume you have them)it can't hurt and may cure the problem?
 
Re: Perplexed!

If the Control leads are run in a metalic (and bonded)conduit, it is probably not outside interference that is causing your induced voltage - it is no doubt caused by the current flowing in your wires - 450 ft is a long way to pass 120 volt ac control or alarm signals. Capacitance of the wires will cause false control or alarm sensing, especially if you are using devices like PLC's that have low amp turn-on thresholds and leakage currents. Long runs of control or sensing applications should be twisted-pair to reduce induced noise. Sheilded twisted-pairs are much better for noise but actually have higher inherant capacitance. For control and alarming apps over 400 ft I always use 24 volt dc (or higher) instead of 120 vac - I have used dc control on conveyor systems that go MILES.
 
Re: Perplexed!

Just did some trouble shooting on a job with a similar problem, The problem was induced voltage on control wiring. The operating voltage on the control wiring was 12 VDC with an operating current of 5-12 milliamp, we were measuring 9 volts AC on the control wiring with the pumps on line and the drives would shut down restart and shut down. We ran a temporary circuit outside the single conduit to the pumps and the problem was resolved. The electrical contractor was instructed to run only one conduit for power and control.

Depending on the type of control panel (operating voltage) this could be a problem and most likely seems to be an induced voltage. Though based upon the distance the fix may be a hassle.

Are there other feeders adjacent to the control wiring?, Is the block heater or other generator operating circuits routed with the controls?
 
Re: Perplexed!

I posted at another site as BobH (couldn't be Bob at this site, already one). Being that it's 34 floors I realize this could be a hassle but why not go with a manufactured cable assembly where the wires are already twisted and spiraled. I'm sure you've seen em. You can order them any awg, length with any number of conductors. I'll bet this would cancel out any magnetic fields on the 6 wires and eliminate your induced voltage issue. But, like I mentioned before, are you absolutely sure this is causing the chaos, or is it just assumed? I've seen induced voltage plenty of times, but I've never seen it cause any problems other than misdiagnosis of something else. Unless of course it's low voltage electronics, the Anti-lock brakes on vehicles have all their sensor wires twisted 3 times per foot to eliminate induced voltage from other sources.
 
Status
Not open for further replies.
Top