Converting a TT earthing system to TNS System

It is in principle 'that simple' but there are a few gotchas.

The connection back to the transformer needs to be able to handle the full prospective fault current - no good having an 'earth' that melts when called upon to operate. It also needs to be wired in a robust way so it cannot go open circuit, but that is probably obvious. In a TT system the earth wiring can be quite thin, as the current is limited by the series resistance of the loop of earth and the entry and exit electrodes. Local wiring codes (or local workers) expecting TT may not call up a suitable cable.

The protective devices (fuses circuit breakers etc ) need to handle that increased current too, and someone needs to think about the voltage rises that occur during a fault. (e.g. if there is an L- E fault and the cable path resistance is equal for L and E, then the voltage at or near the point of fault is half the supply, being divided by two equal resistors, for the short time before it all goes bang and the power goes off ) Simultaneously accessible metal needs to be at the same voltage while this happens. Some thought about where TN-S earthed things and TT earthed things come close to one another may be needed.

What happens if there is a fault to true earth also needs to be considered,  if for example there is a live to water pipes fault, will an RCD or similar operate?  It is possible if the transformer earthing electrodes have a higher resistance to terra-firma than the fault path, that both N and E  becomes several tens of volts offset from the terra-firma earth voltage.

Also are there lightning electrodes to consider, and what do local regs say?

Also where filters and surge arrestors are earthed to matters. You can, and probably should, add additional electrodes to the TN-S earth at such places.

Most of the trickier problems vanish if the system is pure TN-S or pure TT, but if  the strategies are mixed on the same transformer and or the same buildings, then the places where the earths of the two systems meet need consideration.

If it is a private transformer and the site is under technically competent management, it should not be a great problem. It is important that everyone involved realises the significance however.

Mike

It is in principle 'that simple' but there are a few gotchas.

The connection back to the transformer needs to be able to handle the full prospective fault current - no good having an 'earth' that melts when called upon to operate. It also needs to be wired in a robust way so it cannot go open circuit, but that is probably obvious. In a TT system the earth wiring can be quite thin, as the current is limited by the series resistance of the loop of earth and the entry and exit electrodes. Local wiring codes (or local workers) expecting TT may not call up a suitable cable.

The protective devices (fuses circuit breakers etc ) need to handle that increased current too, and someone needs to think about the voltage rises that occur during a fault. (e.g. if there is an L- E fault and the cable path resistance is equal for L and E, then the voltage at or near the point of fault is half the supply, being divided by two equal resistors, for the short time before it all goes bang and the power goes off ) Simultaneously accessible metal needs to be at the same voltage while this happens. Some thought about where TN-S earthed things and TT earthed things come close to one another may be needed.

What happens if there is a fault to true earth also needs to be considered,  if for example there is a live to water pipes fault, will an RCD or similar operate?  It is possible if the transformer earthing electrodes have a higher resistance to terra-firma than the fault path, that both N and E  becomes several tens of volts offset from the terra-firma earth voltage.

Also are there lightning electrodes to consider, and what do local regs say?

Also where filters and surge arrestors are earthed to matters. You can, and probably should, add additional electrodes to the TN-S earth at such places.

Most of the trickier problems vanish if the system is pure TN-S or pure TT, but if  the strategies are mixed on the same transformer and or the same buildings, then the places where the earths of the two systems meet need consideration.

If it is a private transformer and the site is under technically competent management, it should not be a great problem. It is important that everyone involved realises the significance however.

Mike

Thank you very much Mike. I will review the design with these points in mind. Really appreciate your help 

Just a note that, whilst additional earth electrodes may be required to make the TN-S system safe, IEC 60364 series (and BS 7671 in the UK) would require all simultaneously accessible exposed-conductive-parts to be connected to the same earthing system - which means the TN-S earthing system would have to be bonded to the TT earthing arrangement of the TT part of the installation, if there were simultaneously-accessible conductive parts of each system (that is, within 2.5 m).

In this case, because the TN-S is derived from the TT supply, there ought to be no issue with that. It's not always the case the other way round, or with systems having separate supplies.