generator earth advise

[Farmelectrics]

in the building or one of them that we work in we have just had the building surveyed one of the faults that was high lighted was that the generator earth should be seperate from the building earth is this correct what we have is if the building looses power the ups kicks in allowing time for the generator to run essential services ie comms rooms so the bussiness can still operate the surveyer said that if we have a fault current it would take out the lv power and also the generator what he is saying is to prevent this the generator should be earthed to its own earth mat seperate from the buildings has anyone come across this or give us some advice on the subject

 

[PRR]

Not looking at regulations:

There's only one earth and we all share it.

The regs I know best say that Lightning Protection gets its own dirt-rod BUT that is bonded to the electric power dirt-rod and service line. In the case of an alternate power source for the same loads, I do not understand the surveyer's point.

 

[Engineer54]

in the building or one of them that we work in we have just had the building surveyed one of the faults that was high lighted was that the generator earth should be seperate from the building earth is this correct what we have is if the building looses power the ups kicks in allowing time for the generator to run essential services ie comms rooms so the bussiness can still operate the surveyer said that if we have a fault current it would take out the lv power and also the generator what he is saying is to prevent this the generator should be earthed to its own earth mat seperate from the buildings has anyone come across this or give us some advice on the subject

Ask this surveyer how you can isolate the building earth from the generator when it is supplying power to the same building?? You should however have the generators Neutral point earthed, via it's own ground rod(s)/mat.

 

[Farmelectrics]

i dont understand his point either prr another question for yourself or e54 ok are building has its own lighting protection and comes down to various rods in the ground so if the building is struck by lighting it travels down shortest route and disperses into the general mass of the ground why do we then link the lighting protection to the main earth bar surely were giving it a fault path back into the building your thoughts please

 

[lauriehurman]

Well I'm no expert so I may have misunderstood but what you have at the moment is the generator wired as an exposed conductive part - it is bonded to the main earth. When you have an earth fault the body of the generator goes to Line potential (230?) just at the time when you want the generator to start up.

I think (I may be wrong) that the point of giving the generator a "separate" earth is that planet earth is treated as an infinitely large sink so there's no chance of the body of the generator being at Line voltage during a fault.

I've no experience of generators so I may be talking rubbish

Laurie

 

[Farmelectrics]

thanks for your input laurie interesting were still trying to get our head round it ha ha

 

[Engineer54]

Well I'm no expert so I may have misunderstood but what you have at the moment is the generator wired as an exposed conductive part - it is bonded to the main earth. When you have an earth fault the body of the generator goes to Line potential (230?) just at the time when you want the generator to start up.

I think (I may be wrong) Laurie

You thought right, ...you are wrong!! lol!!!

The protection control system should not allow any change-over, when there is a fault existing on the system. Not sure what sort of protection you have on this stand-by gen-set of yours?

There is no bonding involved, only earthing. and seeing as there is no way of isolating the main systems earth from the generators earthing system, and little point even if you could.. I'm still trying to work out his fault current path, as the change-over switch should always be 4 pole on a 3 phase system and DP on a single phase system.

Any half decent mains/generator set-up would incorporate earth leakage protection anyway, where that EFP is located depends on the system hook-up etc...

 

[Farmelectrics]

so e54 in your experience are you saying the generator dosnt need to be on its own earth mat (earth) and could you explain to me why we take an earth from the met to the lightining conductor cos in my mind were bringing a fault back into the building why dont we let the lightining strike just travel down rods and disperse in the general mass of the ground why give it a path back into the building

 

[Knobhead]

The generator must have it’s own earthing provision. You can not rely on the DNO earth only. To do so puts anyone working on the distribution system in danger. They can be linked yes, but a separate earth must be provided.

 

[Rockingit]

My take on this has always been that you need the generator/s and the dno supply to be at zero potential to each other otherwise all sorts of strange and dangerous things could occur. You have a separate rod/mat/whatever for the generator which is derived from a neutral tap which then by default becomes a TT which is why most sets are then fitted with EFP of some flavour or another.

 

[Knobhead]

A correctly installed generator is TN-S

 

[Rockingit]

A correctly installed generator is TN-S

OK, I can see that - so do we just 'ignore' the addition of the rod?

 

[Engineer54]

so e54 in your experience are you saying the generator dosnt need to be on its own earth mat (earth) and could you explain to me why we take an earth from the met to the lightining conductor cos in my mind were bringing a fault back into the building why dont we let the lightining strike just travel down rods and disperse in the general mass of the ground*** why give it a path back into the building

As stated previously, you need to provide an earth rod/mat connection to the generators N-E point/position, which will effectively earth the generators frame as well. And '''NOT'' a 200 ohm affair either!! lol!!

***If only lightning strikes always followed those made paths, and ONLY those paths to ground.. lol!! In reality lightning strikes don't always follow those made paths and can find/seek out other paths, by means of jumping or penetrating...


As for lightning protection, one reason is that the LP system will introduce an extraneous earth situation, say on the roof of a building where different services are present (A/C, Water Tanks, Aerial Installations etc. So to bring the LP to the same potential as the main supply earth, a link or bond is made, normally to the LP down conductor closest to the buildings main intake.

Another reason is that most buildings that support an LT system will be of reinforced concrete construction, (some systems will even use the reinforcement as part of LP the down conductor). It is more than likely that any decent strike on such a building, will reach any reinforcement (normally only around 25mm inside the concrete) anyway.

As this reinforcement will at some point or points be connected to the buildings power supply earthing arrangement, whether it be by a purposely made connection or otherwise it is again sensible to link the LP to the building earthing system. Last but certainly not least, it will effectively stop any jumping of lightning strikes from the LP system to any building earthed metalwork etc!!

The use of SPD's on electrical installations where LP systems are in place, will effectively reduce or even completely remove any lightning strike damage to the system... To be honest, i've never seen any damage caused by lightning strikes to LP protected buildings, that have been totally devoid of such devices, but i would now never not use them, as building system sophistication is increasing exponentially!! lol!!

 

[Knobhead]

In reply to Rock,

No you don’t ignore the rod or mat, it is the origin of an independent TN-S system. Just like the DNO the earths are linked but unlike the DNO systems the neutral can’t be linked, hence the need for a 2 pole centre off change over switch.

This is all to protect anyone working on the distribution system.

 

[Clive P]

Hi,

I've already put my tin hat on! I have always been told and followed the instruction that Lightning protection systems should be totally independent of Buildings or sites Earthing arrangements. I agree with the use of SPD's but this can be complicated and expensive.
To be honest a lot of the work I have done has been Hazardous Areas and the regulations may be interpreted differently,

Regards.