Equipotential Bonding Of Cable Ladders

[jpflying]

In many places cable ladders are earthed even though they ladders contain double insulated, amoured or nonarmoured cables. Other places do not earth these ladders at all. Is there any standards on when to earth these ladders? Is this an issue of touch potential?

Thanks

 

[dpc]

Well, in the US, it is a code requirement. Anything metal should be grounded as a matter of basic safety.

 

[jpflying]

This is not a requirment in Australia. Probably because we use a lot of earth leakage relays on our supplies.

 

[gepman]

jpflying
I don't see how the earth leakage relay will help if cable ladder (I assume this is what we call in the US a cable tray) is not grounded. If you had a "leakage" to the cable ladder and it was not grounded (i.e. isolated) there would not be much current flow since there would not be a complete circuit (ignore capacitive effects). Then if a person completed the path between the cable ladder and ground the current flow may not be enough to trip the "earth leakage" relay but enough to injure and kill the person.

 

[Skogsgurra]

Depending on region, there are different understandings.

In Europe, bonding is not grounding. Grounding is required for safety while bonding is used to minimize ground fault effects on equipment and also people. Bonding is also good for the electric environment since it usually reduces EMI.

There are different requirements for earth leakage/residual current/earth fault/Fi (have your pick) relays. The standard requirement in Europe is 30 mA and less than 100 ms while the US requirement seems to be 10 mA.

I have never quite understood the 10 mA requirement. 10 mA would be very unpleasant, but hardly lethal. Is it because the US line-ground voltage is a lot less than the standard European (230V) voltage so that it sometimes is difficult to have enough "through person" fault current to trip a 30 mA device?

Any views on this?

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[dpc]

In the US, a GFCI device used for protection of people must pick up at 6 ma. But it will take a while(seconds) to trip at this level. At 20 ma to 30 ma, they will trip in around 25 milliseconds.

I believe the basis for the 6 mA limit is based on the "let-go current threshold, which can be as low as 6 mA if you are having a really bad day.

Devices of this sensitivity are only required for situations likely to present a risk to people - bathrooms, outdoor receptacles, etc.

But I don't see why any type of ground fault protection would negate the desirability of grounding and bonding a piece of metal that is carrying insulated cables - no matter how well the cable is made. I've seen plenty of cables damaged during installation into a tray or conduit. I would not want to put all my eggs in the GFCI basket. But then I wear a belt AND suspenders.

 

[gepman]

I agree that grounding and bonding are different (at least as they are defined by the NEC). They are both complimentary in safety.

jpflying should remember that there other potential sources of electrical energy besides the conductors in the cable ladder, in particular portable electric equipment such as welders.

I don't know why the threshold requirements in the US are so low. It could be lawyers.

 

[waross]

You can't run a circuit very far on a North American GFI. They are commonly used for high hazard locations. GFI breakers are available, but GFI receptacles are commonly used.
You can run a lot more field wiring on a 30 ma GFI breaker than you can on a North American GFI without nuisance tripping.
It was always my understanding that the North American philosophy was to protect any and all idiots who may use their bodies to complete a circuit to ground, and the European philosophy was to detect insulation failure to ground or low impedances to ground.

Does this sound like a reasonable assessment?
North American;
Pro> Very good life protection.
Con> Only selected high hazard circuits are protected.

European;
Pro> More widespread coverage. Many circuits are protected that would remain unprotected in the US.
Con> If you do get a shock, 30 ma at 230 V will shake you up a lot more than 15 ma at 120 v.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[dpc]

Bill,

I believe 30 mA GFCI breakers are allowed in the US for protection of systems such as heat tracing, but are not allowed for people protection where GFIC protection is required by Code.

I think another source of difference are the different methods of system grounding. And of course, each method is felt to be the "best" by someone. I have a Schneider white paper that discusses various grounding methods used throughout the world, and it was "self-evident" to the French authors, that the US method was obviously inferior to the French system. I can never keep all those "TNC" designations straight so they could be right for all I know.

As far as physiology, ventricular fibrillation can occur around 70 mA or so. Respiratory paralysis can occur around 18 mA (60 Hz), but it isn't going to kill you unless it goes on for a long time. The GFCI pickup is supposed to be below the "let-go" current for women, which is about 6 mA, worst-case.

BTW, I can attest to the fact that GFCIs do a great job of protecting not only idiots, but incredibly "intelligent" people such as myself - and no I don't want to talk about it.

Cheers,

Dave

 

[itsmoked]

I only ever hook up with non GFI circuits.

Keith Cress
Flamin Systems, Inc.-

http://www.flaminsystems.com

 

[gepman]

Regarding this international grounding issue, is there a standard European code for electrical installations similar to the NEC in the US? Is there a link or a way to purchase a copy written in English?

Also since it sounds as if cable tray (in some countries) does not need to be grounded due to the circuits being protected by "earth leakage" circuit breakers, does that mean that motor circuits, transformer circuits, and all other similar power circuits are protected by "earth leakage" circuit breakers?

 

[dpc]

gepman,

If this long link works, you might try this:

http://www.schneider-electric.com/d...ility-safety/low-voltage-minus-1kv/ect173.pdf

Just have your BS detector handy.