Equipotential zone

[Jako13!]

Can someone help me with my understanding of this. EPZ zone within an installation should all rise to the same potential when an earth fault occurs so any where outside the zone would be zero if it’s the ground outside. So in theory shock should not be possible within the EPZ? Am I correct with my understanding?

 

[Ian1981]

Yes you can still receive an electric shock but the idea is that you can’t touch two metallic parts ie exposed to exposed, exposed to extraneous , extraneous to extraneous that are at different potentials and receive a greater potential difference across the two parts resulting in a greater electric shock.

 

[Jako13!]

Yes you can still receive an electric shock but the idea is that you can’t touch two metallic parts ie exposed to exposed, exposed to extraneous , extraneous to extraneous that are at different potentials and receive a greater potential difference across the two parts resulting in a greater electric shock.

I just can’t get my head around that just as if everything is at the same potential there is no voltage to push any current through you. It’s like when people say they’ve had a shock off a fibreglass ladders when they are isolated from the earth ?

 

[DPG]

I just can’t get my head around that just as if everything is at the same potential there is no voltage to push any current through you. It’s like when people say they’ve had a shock off a fibreglass ladders when they are isolated from the earth ?

No, the exposed metal parts are all at the same potential. But you can still get a shock if you touch a metal part and a live conductor.

 

[pc1966]

As above, the idea of an equipotential zone is that everything is at essentially the same voltage.

Where 'everything' refers to significant touchable conductive parts, and the 'same' means in practice a low enough difference so you don't get a dangerous shock (so typically less than 50V, or maybe 25V for wet areas, agricultural, etc).

In practice of course all bonding has some impedance, so at high fault currents you won't get zero volts but if designed and constructed properly then you won't be exposed to a significant risk, either long term if is a small current, or short term as the OCPD acts (the whole EEBADS idea).

Also in practice you won't bond everything, partly as much of the area is not going to induce a voltage difference, and partly just the practicalities of what can sensibly be bonded and what is at risk of being touched, etc.

To induce a voltage you need two objects that are normally at earth potential, but under fault conditions they are different. The obvious case would be the metal enclosure of something if it has an internal fault L-E, then you hope the CPC diverts the current until it is cleared. The other case (the risk from TN-C-S) is that a major fault raises the local CPC potential with respect to the Earth, and you have some extraneous part that allows you to see that voltage difference.

By bonding two parts you force them to be close in potential, either pulling down the live one, or pulling up the "earth", in either case the goal is not to see any big difference.

With the likes of a fibreglass ladder it is almost certainly static electricity. That usually arises from friction (the triboelectric effect).

 

[Jako13!]

As above, the idea of an equipotential zone is that everything is at essentially the same voltage.

Where 'everything' refers to significant touchable conductive parts, and the 'same' means in practice a low enough difference so you don't get a dangerous shock (so typically less than 50V, or maybe 25V for wet areas, agricultural, etc).

In practice of course all bonding has some impedance, so at high fault currents you won't get zero volts but if designed and constructed properly then you won't be exposed to a significant risk, either long term if is a small current, or short term as the OCPD acts (the whole EEBADS idea).

Also in practice you won't bond everything, partly as much of the area is not going to induce a voltage difference, and partly just the practicalities of what can sensibly be bonded and what is at risk of being touched, etc.

To induce a voltage you need two objects that are normally at earth potential, but under fault conditions they are different. The obvious case would be the metal enclosure of something if it has an internal fault L-E, then you hope the CPC diverts the current until it is cleared. The other case (the risk from TN-C-S) is that a major fault raises the local CPC potential with respect to the Earth, and you have some extraneous part that allows you to see that voltage difference.

By bonding two parts you force them to be close in potential, either pulling down the live one, or pulling up the "earth", in either case the goal is not to see any big difference.

With the likes of a fibreglass ladder it is almost certainly static electricity. That usually arises from friction (the triboelectric effect).

So in theory in the EQZ the voltage between metallic parts is the main issue but what about hand to foot contact? Obviously standing on the general mass off earth and touching a faulty appliance within the zone is going to give you a whack but what’s the science when you’re within the EQZ?

 

[pc1966]

So in theory in the EQZ the voltage between metallic parts is the main issue but what about hand to foot contact? Obviously standing on the general mass off earth and touching a faulty appliance within the zone is going to give you a whack but what’s the science when you’re within the EQZ?

Well you want the main earth to be low enough impedance so any fault that causes a CPC current will disconnect before you are exposed dangerously.

In most domestic/office spaces you won't fine contact with the Earth beyond that provided by the classic "extraneous conductive parts" such as gas/water pipes or steel structural work, all of which should be bonded (of course there may be things that get overlooked like telecoms / antenna cables). But in outdoor work, agriculture, and some industrial sites that is a very real possibility.

Hence some of the special area regulations (e.g. no PME for ships or caravan supplies, etc) and the shorted ADS times for TT where a fault can raise the CPC "earth" to a much higher potential w.r.t. true Earth during the disconnection period.