MV Fault Loop

[Cookie]

I've been thinking, do disconnection times apply to MV? Say a fault occurs on the MV system. This would result in 3kv from the MV neutral to remote earth. In theory this voltage would end up on the LV neutral, enter the structure, cross over to the MET in a TN-C-S supply and then energize all metal connected to the CPC to 3kv. If someone is standing outside and thus away from the bonded foundation using a grill or none double insulated power tool would they not be hit with 3kv? How is this safe? I genuinely wonder about this.

 

[plugsandsparks]

I agree, any MV fault to earth is a serious risk, but well understood by the distribution companies, so these types of faults have ADS. I would expect them to operate their equipment to limit the risk to the LV network and local earth systems to an acceptable level, I dont know what they consider acceptable but i dont give it much thought in my day to day activities.

 

[Cookie]

Same. I wonder what the required ADS is for DNOs, and how they "control" the elevated voltage on the LV PEN condutor.

 

[R-fur]

Its no different in principle from LV. There are disconnection times which vary depending on earthing systems. The higher the voltage to shorter the time.
The rise in potential is controlled by bonding.

 

[Cookie]

Alright, but I think it would be difficult to bond outside a structure.

 

[Cookie]

Its no different in principle from LV. There are disconnection times which vary depending on earthing systems. The higher the voltage to shorter the time.
The rise in potential is controlled by bonding.

I have to disagree, it seems like Uf can not be controlled through bonding.

 

[pc1966]

The local voltage differences (i.e. what the operator during maintenance experiences) is controlled by bonding of things.

The LV impact is controlled by the earthing arrangement, another thread had the values but basically it is either physically separated HV & LV earths, or if combined then it has to be a very low impedance of 1 ohm max.

I have a funny feeling that I read the combined earth might not be done in new substations any more, but others with more DNO knowledge might be able to expand on that.

 

[R-fur]

The HV earth is connected to the transformer tank, the HV switchgear and the LV cabinet, there is also a conductor buried at shallow depth in front of the HV switchgear where the operator would stand.
The LV earth system should be at least 8 meters away and is brought in on an insulated conductor to the neutral bar.
In the event of a fault all the exposed metal work in the sub is bonded and the risk of shock is minimised. The chance of any rise in potential being passed to the LV earth 8m away is minimal.
On an overhead system the HV earth runs down the transformer pole and the LV earth is one span out from the transformer.

 

[Cookie]

The HV earth is connected to the transformer tank, the HV switchgear and the LV cabinet, there is also a conductor buried at shallow depth in front of the HV switchgear where the operator would stand.
The LV earth system should be at least 8 meters away and is brought in on an insulated conductor to the neutral bar.
In the event of a fault all the exposed metal work in the sub is bonded and the risk of shock is minimised. The chance of any rise in potential being passed to the LV earth 8m away is minimal.
On an overhead system the HV earth runs down the transformer pole and the LV earth is one span out from the transformer.

Ok, so the MV and LV neutral/earth is separated.