suspected faulty 6.6kva transformer

[Danspark]

I think I have a faulty 66kva transformer but have little to no experience with them (delta star I believe)

Call out due to reported problems from some of the tenants (offices that are rented within part of the building)
some RCD protected ring final circuits disconnecting. Investigated at the local sub-main distribution board and discovered some strange goings-on... The voltage of all three phases with reference to each other and with reference to neutral are as you would expect. When L1, L2 & L3 were tested with reference to earth I have readings of 130, 290 & 380 volts (approximate values) the voltage from earth to neutral is 130 volts.

I toddle off to the factory side of the installation and when on site I am greeted by "electricians" scratching their heads and removing RCD socket outlets and replacing for standard.

I make my way over to the substation with a privately owned 6.6kva transformer. The 1st part of the installation is a bus bar chamber. I gingerly remove the lid and see that one of the 3 phases is shorting with the neutral... Booyah; I've found the problem. I isolate the supply, separate the phase conductor from the neutral conductor and re-energise. I test the installation, pat my self on the back and go home.

Today I get called out due to the same symptoms. I go to the original distribution board within the offices and discover that the same problem exists. Back over to the substation I go and the same readings are observed. I gingerly check the neutral conductor is mechanically sound and i'm satisfied that it is. The earthing arrangement inspires confidence to, 2 earth rods within enclosures, a good connection to the casing of the transformer and a sound connection to the isolation switches and busbar chamber

maybe one for Engineer54? ...if you're not to busy bashing partP and the NICEIC

Thanks Rob, I'm not sure of the size of the transformer

Edited for simplicity:

the secondary side of my delta star transformer appears to generating 3 phase perfectly and a good, mechanically sound, centre tapped neutral. the voltages I observe with reference to earth (across each phase and to neutral) would suggest that the earth is poor. But how can it be with the following arrangement; 2 x earth rods to side of transformer, earth connection on casing of transformer, earth bar running the length of the wall (about 12 inches off ground and showing signs of discolouring (not sure what this is for)) and 10 earth cables leaving the room to various extraneous conductive parts ie; gas, water, structural steel etc..

And why would the installation remain energised while L3 has been sat in contact with the neutral conductor?

 

[Danspark]

bumping cos it needs bumping

 

[telectrix]

you may have a bit of a wait as the forum has been offline a lot over the last day or two. some of the guys may not log on for a day or so.

 

[Danspark]

thanks man, ill be patient.

 

[Bobster]

6.6kva is tiny fella. I'm sure that's a mistake.

Have you done any electrical testing yet? Sounds to me like your phases are still floating, probably a high resistance N->E joint.

 

[Richard Burns]

The Beama RCD handbook covers a lot of information about RCD tripping, but I think RoB2 has it down pat really. From your readings there is a N-E potential, though it is surprising the L-N readings were OK with a line to neutral short.

 

[Lucien Nunes]

Agreed, there is a red herring there. Either you made a mistake logging the voltage readings or that wasn't a real short, otherwise it would have gone bang or taken out a fuse. The problem does seem to be that the neutral isn't earthed at the star point. Or perhaps I should say, *a* problem, as there might be more to it than meets the eye. Find out how it is meant to be earthed...

 

[Danspark]

No mistake with my collecting voltages.
It certainly was a short, but why it didn't go bank or poof has me very puzzled
neutral seems fine.. the problem is with the earth, but i don't understand why. please see my edit at the bottom of my original post. still need your thoughts!

 

[Lucien Nunes]

It has to obey the laws of physics - you cannot have mains voltage across a dead short. So either the L-N voltage at that busbar was zero, or it was not a short. When you are measuring L-E and getting funny readings, presumably you are using the CPC instead of poking your probe in the mud. Thus the earth rods etc are not part of that test, what you are finding is that the neutral point is not connected to the MET. Have you checked this connection?

 

[DPG]

Can't offer much help but agree with the above, how can you have a correct L-N voltage when L is shorted direct to neutral??? Daz