Ground or Spare Unused Conductors in a Cable 2

[polareng]

I am getting conflicting discussion regarding grounding unused/spare conductors in a cable. My VFD manual Power Wiring section says unused wires in a conduit must be grounded at "both" ends to avoid possible shock hazard caused by induced voltages.

Now granted I am universally trying to apply this across the board whether power, control, or signal wiring and also to unused conductors within a cable.

Am I going overboard? What I have been told is to just spare-out or insulate the ends of the unused leads. I could go into my philosophy of why I would think you should at least ground one end but I am more interested in the proper typical good shop practice and anyone elses philosophy.

 

[waross]

I would avoid the shock hazard by insulating both ends.
respectfully

 

[dpc]

As a general rule, I think trying to ground all the spares would lead to more safety issues that it would solve. PWM outputs to motors may be a special case due to the high-frequency components. For low-voltage 50/60 Hz, I agree with waross that just insulating the ends should be adequate.

 

[oldfieldguy]

In the electrical transmission business, we typically grounded unused conductors on long runs of cable in the 230 kV and 500 kV yards.

We didn't make a big deal about it, just stripped back the unused conductors long enough to make up in a split-bolt connector and tied it to the grounding conductor in the cable trough.

old field guy

 

[HCBFlash]

In my 28 years, when dealing with ordinary feeder and branch circuit types of wiring, I've always seen spares taped at the ends, or very rarely used for "suplemental" grounding conductors where some concern of fault existed.

In instrumentation, ground loops can be an issue.

You could not go wrong by making a policy of connecting all unused conductors to the grounding conductor at the feed end of the cable or raceway, and fully insulating the other end. Obviously spares need to be clearly identified at all boxes or fittings.

I'd not be worried about grounding both ends, but run across enough situations and incidents where "taped-off ends" weren't insulated very well and problems arose.

Keep in mind that wet conductors are trouble, and that is often at the root of problems encountered with spares.

I think setting a sound standard, enforcing it, and insisting that spares be treated at least as well as any "important" conductors is itself a good idea


Please let us all know how this works out for you! and don't forget faq731-376

 

[polareng]

HCBFlash--Can you elaborate a little more on the problems that arose when "taped off ends" weren't insulated very well and what kind of trouble you experienced with wet conductors?

I am trying to convince others of this issue, but I am not getting the agreeable response as I would have expected. My main point has been that if the cable with spares happens to be run close to high voltage cables, since we operate with a lot of temporary equipment in an heavy industrial environment, the high voltage would induce a charge on the spare conductors which could be unexpectedly discharged by personnel if they were not aware of the situation when they decided to use the spares for whatever reason. Although, I haven't heard of anyone being shock in this type of situation, I have heard of cables holding a charge for some time well after meggering.

 

[dpc]

Are you talking about low voltage or high voltage cables? Low voltage cables really should not be run "close" to HV cables - in fact they should be in separate conduits or separated by a metal barrier.

Any conductor, including "ground" conductors should be considered energized until proven dead. In many control rooms and electrical rooms, there are hundreds and hundreds of spare wires. Any assumption that these will all be grounded for safety purposes is going to lead to a false sense of security.

 

[polareng]

dpc--I absolutely agree on all accounts, but in an industrial environment I can not rely on others to be aware of my cable concerns or of necessarily performing voltage checks on spares. There are literally hundreds of temporary power cables, as well as control cables, which are not typically run through conduit, running everywhere that are not my cognizance so I was just trying to justify some level of safety in case someone is not performing thier voltage checks or routing low voltage away from HV cable.

I'm just saying sloppy work happens and therefore an accident could be prevented by performing certain good shop practices such as grounding at least one end of a spare conductor, then it becomes a non-issue if they happen to forget their voltage check. I'm not say this is okay, but is a backup safety practice.

 

[dpc]

I can see your logic, but then if someone is shocked because they forgot to test the "Spare", it becomes someone else's fault because the spare wasn't properly grounded.

I'm not saying it is wrong to ground the spares, but on balance, I'm not convinced it would contribute much to overall safety. Most electricians are electrocuted by circuits that they "know" are dead, not ones that they think are probably dead.

With high frequency power circuits, there can be RF voltage on a cable that is grounded at both ends. Probably not going to kill anyone, but it can sting (speaking hypothetically of course).

 

[HCBFlash]

The problems I've frequently encountered with spares being just taped off is that they're given no thought, and the tape comes loose, doesn't exclude water, becomes "hot" by leakage through insulation or tape damage, etc..

Wet cables/conductors get that way by leakage, and leakage is the real problem. It again proves that the insulation has failed. I frequently use these little "touchless power sensors" (or whatever they're called) that have an annunciator of some sort that lights, or hums, or squeaks..... but get people hurt when wet cables are involved. I'm no EE, (so maybe I don't belong here) and can't intelligently discuss oddities of inductance caused events, but water always complicates and exacerbates these things.

I'm sure you can imagine the galvanic issues of wet conductors also. Unless it is absolutely certain that a wet conductor got that way from a specific incident and the insulation is entirely uncompromised, it is suspect and may well cause trouble.

At least that's been my experience, and that of others I've worked with.



Back on the ungrounded theme though, safety is a large factor, but not the entire story.

My early experiences with ground loops was in audio microphone circuits. When you've got amplification and can hear the signals induced on ungrounded, or grounded through connections of discernable impedance, you get a different view of what's going on in these situations.

Perhaps someone reading this thread can tell us some things about induced loads in/by non-magnetic conductive objects. If we're dealing with high-speed switching of D.C. to essentially simulate a somewhat sinusoidal A.C. wave, we know that high enough frequencies are involved to fall into or near the range of RF. We know that RF requires power to generate it, and that power can be induced in antennae by it. I know that an electrician in a wet environment can REALLY feel the RF power in a conductor that's using him as a ground, or at least ground reference.

It strikes me that if the inductive load (of the unintended antenna- a.k.a. "spare") on the power conductors is uneven, like primarily 1 of the 3 phases, then there will be repercussions in the other 2 phases as well.

 

[HCBFlash]

polareng, I'm wondering about what you referred to in your initial post :

" .....I could go into my philosophy of why I would think you should at least ground one end but..... "


something beyond your mention in your other responses above? I too am (at least presently) working in an environment with cables everywhere. 13.8Kv shielded power cable on down to telephone and analog sensors. Many (almost all signal) have spare conductors. Spares in connectorised cables are simply cut-back flush with the jacket, and left. Most instances of spares in junction boxes or cabinets are taped and bunched up, or tied up in a corner of the cabinet. Occasionally spares are landed on spare terminals. Usually this is no problem,,,, until there's trouble somewhere in the system and it's troubleshooting time.

Big problems can occur when someone wants to use one of these spares. When I'm given responsibility on nearly any project I've set a policy (on power wiring) of using spares "as supplemental grounds" and clearly marking them "suitable for spare". Connectorised cables are the problems, and if I had any say I'd use them as additional grounding conductors.

Control cables I try to terminate both ends and jumper the terminals to ground, similarly marked. Phone, UTP network, occasionally analog signal, I use the old-time Bell System standard of wrapping (a useable length) of the spares back around the end of the cable and taping over the finished cable end (at devices where punch-downs aren't there).

I really don't think that all spares in all cables and raceways" can be handled the same. Spare multi-conductor cables in cable trays, bundles etc. I almost always insulate, waterproof, and label each end.


:-0



.... WOW! It almost looks like somebody fired up a spare inside HCBFlash!

 

[Marke]

The big issue when you are dealing with VFDs, is that the output voltage is made up of a high frequency PWM waveform with rapid switching edges.
Unused conductors in the same cable, will have a small amount of capacitance to the output conductors.
When we consider the normal line frequency, there is no real issue, but when you look at the output voltage waveform of the VSD, this small capacitance is a major problem. The current through a capacitor is related to the capacitance and the rate of rise of voltage, and the peak voltage. The switching wavefronts are usually less than 1 V/uS and so even a few pF will result in quite a high current compared to line frequency.
If you allow the cores to float, they will have a high voltage high frequency waveform capacitively coupled and this can do damage if it comes into contact with ther equipment, or people. It is best to earth the spare conductors to prevent the build up of high frequency voltages on these cores.

Best regards,

Mark Empson

http://www.lmphotonics.com

 

[armandh]

if you are writing rules
spares in sensor wiring [ground only at the receiving end]
spares in a cable carrying over X volts should be grounded
low voltage control [ungrounded insulated and marked spare]
set your rules to the needs