TT system, fault current and neutral line voltage

[hansforum]

Hi!

In the attachment a posted picture with the schematic that I will talk about.

I wonder what will happen with the neutral line voltage at a transformer when large fault current starts to flow. For example if in someone's house hot line touches metal object that is conected to the ground line and that house doesn't have RCD protection (only circuit breakers) fault current starts to flow towards earth. If on that line circuit breaker is very high rating that fault current is not high enough to trip it and if we assume that Ra=Rb there will be pretty high voltage accros Ra and Rb.

Does it mean that while fault current is flowing neutral to earth voltage will rise from 0V to voltage across Ra? Does this mean that all homes that are connected to this transformer will have deadly voltage on neutral line?

One more thing, how big is the potential gradient around earth electode in event like that and is it high enough to be deadly?

 

[Bramb]

Hi,
you should check IEC 364-4 ("Electrical installations of buildings part 4: protection for safety")for safety requirements of houses electrical circuits.

Ciao
Erminio

The difference between overload and short circuit lies in the nature of the fault, not in the value of the current.

 

[hansforum]

Thanks for your suggestion but unfortunately I don't have access to those papers.

Is there somebody who can answer my question?

 

[ScottyUK]

Your assumptions are basically correct.

For the benefit of non-European readers, the TT earthing system is "Terra -Terra", using two discrete earth electrodes with one at the source and one at the load and with no metallic interconnection between them.

Yes, for the fault you describe there will be a rise of earth potential for the installation relative to a remote earth. This is one reason for the use of equipotential bonding within the installation because this prevents a touch voltage between conductive objects in the immediate area of the installation. TT systems generally require an RCD in the UK, unless they have an unusually large earth electrode.

The step potential is unlikely to be of major concern provided the fault is cleared quickly. The earth electrode and ground conditions have a significant influence on the voltage profile.


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