Transformer Neutral Grounding

BS EN 50522:2011 gives requirements for determining if the HV and LV Earth

nests can be connected together and to determine the combined HV/LV earth

resistance.

Very interesting subject.

Having examined the results of lightning strikes and ground surges thru the years i am concerned about coupling all equipment to a common earth.

In some countries they fit a series impeadance between the neutral of transformer and earth with a substantial surge divertor a a backup.

This device is monitored and can warn of failing insulation in transformer and circuits.

My concern is that there are serious ground surges, if lightning has struck nearby and all the equipment because it is coupled to a common ground will be affected by the surge.

I do not know what the full answer is and feel the really knowledgeable engineers should put some thought into this question.

To earth everything sounds safe and sensible but can be causing unnecessary damage.

BS7671 doesn’t encompass distribution transformers.

For standard LV transformers the neutral/earths can be linked. Where there is a restriction under ESQCR is linking the MV and LV earths where the LV earth must be under 1Ω for them to be connected together. Above 1Ω there has to be an eight metre gap between the earth nests.

BS 7430:2011/Annex A Guidance on typical HV/LV interfaces

A.2 Legislation

The Electricity Supply Regulations (replaced by The Electricity Safety, Quality and

Continuity Regulations in 2002) required that where in a substation the HV

equipment earth and the LV neutral earth were common, that the resistance to

earth has to not exceed 1 Ω. For most substations this value was provided by the

un-insulated protective sheaths of the older types of cables in use and was

normally sufficient to lower the impedance of these cable sheaths to ensure

sufficiently low earth potential rise (EPR) for general combination of HV and LV

earth systems even with very high earth-fault current. However this simple

requirement is no longer adequate.

Regulation 8(2) of The ESQCR [1] requires that in respect of any high-voltage

installation, the earthing has to be designed, installed and maintained so as to

prevent danger in any low-voltage network occurring as a result of any fault in

the high-voltage network. The advice in the Guidance on the ESQCR [1] is:

The current advice of the Health and Safety Executive is that touch voltages

should not exceed curve the recommendations of BS EN 50522, National Annex

NA.2.

A.3 Combined HV/LV earth resistance

BS EN 50522:2011 gives requirements for determining if the HV and LV Earth

nests can be connected together and to determine the combined HV/LV earth

It is not clear what kind of system is provided and what is the purpose of the connections:

PE,PE-N,N according to which system. Is the earthing bar EB-LV-101[102] connected directly to an earthing electrode?

In any case the neutral of transformer secondary has to be solidly earthed-preferred directly-See for instance BS7671[ 4.2.3.4 TN-C-S system for instance] or IEC 60364-5-54.

However TN-C-S system-in my opinion- it is the best system but required multiply earthing points along the PEN run.

And what did your designer tell you to do? The drawing is clear how it should be done. Why don't you get onto site and see how it is done rather than asking here. Do you obtain your working experience on this site too?

As a general rule-of-thumb, the ground and the neutral have separate purposes and their own currents associated with them. Therefore, they should have their own conductor to accommodate them (regardless if they're tied together somewhere else). If the ground safety conductor needs to carry fault current away from somewhere, it needs the full capacity of its conductor. It cannot be sharing normal load current of the neutral conductor, and vice versa, and negate the functionality of the conductor(s) that's supposed to provide that functionality.