explanation of main earth bonding?

[Kev2632]

Hello, could anybody explain why Main earth bonding is required and how it works in an event of a fault?

 

[La Poste]

There may be a substantial potential difference.

Could you explain further?
Why would there be a substantial potential difference?

 

[Deleted member 9648]

There may be a substantial potential difference.

As above...please explain how there can be a substantial PD between CP's and ECP's when main bonding has been carried out.

 

[Chr!s]

As above...please explain how there can be a substantial PD between CP's and ECP's when main bonding has been carried out.

Ut= If R2

 

[La Poste]

So the idea is to get R2 as low as possible, recommended as 0.05 Ohms, well according my 2391 lecturer.

This will be the concept of touch voltage that I don't know much about then.

Is that right?

Also I understand bonding being needed to create an equipotential zone and with a TN-C-S system it is important because of the risk of a lost or high resistance neutral joint but are there other instances where a TN-C-S earthing system floats above true earth?

I mean if my next door neighbour had an earth fault and seeing as I am connected to them via my neutral will the metal work in my house rise to the same potential as his assuming we are on the same phase?

Basically can anyone share any knowledge of when a TN-C-S earthing system would float above 0 volts?

Thanks.

 

[Chr!s]

So the idea is to get R2 as low as possible, recommended as 0.05 Ohms, well according my 2391 lecturer.

This will be the concept of touch voltage that I don't know much about then.

Is that right?

Also I understand bonding being needed to create an equipotential zone and with a TN-C-S system it is important because of the risk of a lost or high resistance neutral joint but are there other instances where a TN-C-S earthing system floats above true earth?

I mean if my next door neighbour had an earth fault and seeing as I am connected to them via my neutral will the metal work in my house rise to the same potential as his assuming we are on the same phase?

Basically can anyone share any knowledge of when a TN-C-S earthing system would float above 0 volts?

Thanks.

The voltage dropped across R2 will determine the PD, the PD may be quite high on stiff supply's.

 

[La Poste]

The voltage dropped across R2 will determine the PD, the PD may be quite high on stiff supply's.

Sorry but what is a stiff supply?

 

[Chr!s]

Tnc-s supply Ze= .05 R1 = .1 R2= 1

If= 200

Ut = 200 x 1 = 200 volts

HTH

 

[La Poste]

Right I understand.

I thought originally you were talking about the potential difference under fault conditions between two exposed or extraneous conductive parts that had been bonded together.

 

[Chr!s]

Right I understand.

I thought originally you were talking about the potential touch voltage under fault conditions between two exposed or extraneous conductive parts that had been bonded together.

i am

 

[La Poste]

Why would R2 be 1 ohm, surely it would lower than that?

 

[telectrix]

grossly undersized cpc compared to 0.1ohm for live conductor?

 

[Chr!s]

grossly undersized cpc compared to 0.1ohm for live conductor?

Ok, i dramatised it a little

Change it to .25, its stil 140 odd volts

 

[La Poste]

If you have a long bonding cable to a pipe in the proximity of a far reaching metal clad socket at the end of a long radial the potential volt drop between the two under fault conditions could be quite high if the earth fault is right at the far reaching socket and you happen to touch both the pipe and socket simultaneously during the time of the fault.

Probability of this happening is probably :detective: very small but it could happen.

Supplementary bonding.

Can you supplementary bond an incoming metal pipe to a distant metal clad socket, is it a good idea?

Thanks.

 

[CrazyScozzese]

Is it simple. If all the bonded metallic parts within simultaneous touching distance are at the same potential, you don't get a shock in the event of an earth fault, or close potential, and in theory you don't get time to die before the protective device cuts in. So when they say it reduces the magnitude, the magnitude is the difference between two extraineous metallic parts.

Earthing reduces the duration, a good earth means more current flows and so the protective device trips sooner.

Bonding is joined to earthing because it provides a combination of the two, pulling the touch voltages down to earth potentials and providing parallel paths to expedite tripping.

Sup bonding where required is not necessarily connected back to the met directly, but should be connected to the CPCs.

These do have downsides of course! See earth free zones, etc.