300 amp genarator

[unclebubble]

Does anyone know the expected Ze reading I should be looking for from a 415v diesel powered generator.Basically we have been called in to test some distribution boards(7 all in all which come off one main dist board fed via generator)And none have earth rods at all.The only one that does have an Earth rod is the main dist board and I'm getting 40 ohms (all phase/n/e)I need to know for periodic
The Dist board is 3 phase and the main isolator switch is 125 amps.Also I'm pretty sure the main switch controlling everything should be a c type RCD.The thing is they have a lot of welders/and steel bending machines that I'm concerned would trip an RCD out with the initial surge.
This is a permanent generator.Any help would be appreciated

 

[Spark123]

Is the star point or neutral of the generator grounded? If not then it isn't a T system (as in TN-S), therefore not EEBADS. Can you draw a diagram of your earth path?
What is a Type C RCD?

 

[Lectrician]

No mains supply on site? Gen used as primary supply?

I would expect to see the the output of the gen present the three phase and neutral PLUS an earth which will be derived from the centre point of the alternator (neutral).

You will in effect then have a distributed TN-S system - The earth being seperate from the neutral through out the distribution and installation.

On a generator that is capable of providing 300amps per phase, the internal impedance is going to be negligable, and therefore the Ze likely to be less that 0.1 of an ohm - TBH I would bet on less than this, to the point where your meter cannot actually read low enough. The PFC will almost certainly also be above the 20kA readable by most meters.

There is no requirement for an RCD on a genset - The earthing and bonding should be done correctly, and should therefore form an EEBADS environment.

The only earth rod you should have should be from the gens earth terminal to a rod - this puts the earth and neutral to 0v with reference to mother earth. This earth rod should be tested using an earth rod tester - no way you can use a loop tester for this.

Can you describe a little better what you actually have there?

 

[Lectrician]

Is the star point or neutral of the generator grounded? If not then it isn't a T system (as in TN-S), therefore not EEBADS. Can you draw a diagram of your earth path?
What is a Type C RCD?

What do you mean by that? That doesn't make sence.

 

[Spark123]

T as in terre as in the star point being earthed.
If it isn't earthed then it isn't a Terre system i.e. it cannot be TN-S which I would expect to see a generator of that size.

 

[Lectrician]

Yea, just confusing the way you wrote it really......T.......then saying TN-S in the same sentence.

If it has no reference to earth, it is not even a T system - It is an I system.

 

[Spark123]

I see what you mean
T = Terre, N = earth connection via the network, S = Separate.
Seems odd to have an IT system, I have only ever seen one which was in a test area, not really one of my strongest points!! Does IT still require monitoring for first fault condition?