Earth fault on a TN-S system

[Julie.]

Your cable did not get damaged by the 126A setting, this is just the setting, just as the 346A is a setting.

The actual current will be determined by the voltage and resistance (cable + external [Ze])

So if you measured the total resistance as 0.23ohm (not unreasonable) with 230V - the ACTUAL fault current would be around 1kA no matter what the setting is!

Your cable must be able to withstand this, irrespective of the protection settings.

The type of protection (not settings) will determine how much damage is passed through - if you have a class 3 50A b curve 10kA mcb then the I^2t let-through will be circ 135000 A^2 seconds for high currents (above the instantaneous /magnetic value)

If you have mccbs you need to check the cable adiabatic withstand against the let through of the mccb

 

[davesparks]

The cable downstream the 50A breaker and the trip on 630A breaker

We understand that it is a cable that has been damaged, whar we are asking is what the damage actually is? For example is it burned at its terminations, melted at some point?

 

[Marcopolo]

We understand that it is a cable that has been damaged, whar we are asking is what the damage actually is? For example is it burned at its terminations, melted at some point?

The damage was on the terminations.

 

[davesparks]

The damage was on the terminations.

Do you have any pictures you could show us?

 

[Marcopolo]

Do you have any pictures you could show us?

I have no pictures, however it seems the cable is damaged along the way, not at the terminations. Sorry for misleading.

 

[DPG]

I have no pictures, however it seems the cable is damaged along the way, not at the terminations. Sorry for misleading.

Hard work this.

 

[Julie.]

I have no pictures, however it seems the cable is damaged along the way, not at the terminations. Sorry for misleading.

We are struggling here trying to understand the situation.

Your protection settings are not coordinated, but that sounds like the least of the problems.

I understand the equipment is directly off a transformer - but we don't know the size.

Quite often the fault current at this point is well in excess of the level most people are used to, at home my fault current is a little over 1kA, direct on a medium sized transformer it could be in excess of 20kA, certainly this is way more than the usual mcb can handle, but within the range of acbs and mccbs.

The design of cables and connections for this type of fault current is not what many people are used to, for example, the normal size cable for 40/50A could be 6 or 10mm2, but this size of cable would not withstand fault levels of this order, the only suitable protection would be fuses as they limit the current with a fairly small let-through enabling the cable to survive, with mccbs at this fault current, one has to size the cable to withstand the let-through which is considerably more for a mccb than a fuse.

You need to check your perspective fault current and see if the cables can withstand that.

Protection settings have little impact at high fault currents as the mccbs are in fault limit mode, and have a let-through fixed by type of mccb

 

[Pete999]

Hard work this.

without any pictures, it's difficult to form an opinion

 

[Marcopolo]

I know, I am waiting for some pictures myself.

I am actually wondering if on this configuration TN-S, an earth fault protection is really needed.