Resistance of a transformer windings.

[LeeH]

Does any of you clever chaps know what the resistance of a 5kVA transformer should be? It's a 240-110 CTE.

I have a resistance of 0.16 Ohms (1400A)

If I recall what little transformer theory I covered years ago the impedance will increase when energised but how much I have no idea.

Basically we did some remedial work on the main section board at our Manchester branch and the supply to the transformer won't hold in.

The OCPD is too small, it's a hager C32. Hager recommend a C63 or a D32 for a 5kVA so that's obviously a problem but it's been fine for years.

 

[Rpa07]

What, if any, would an isolator to start with the 'early make, late break' set up on it make?
What would industry use as a soft start in this situation or is it down to the 'curve' on the breaker?

 

[PEG]

I do not believe an isolator will assist,but there are industrial controls,which serve this purpose.

Some transformers come with these fitted,or you could try fitting a suitable NTC thermistor,which i have,on a very old 3.3,which has powered everything and not wiped out even a 20A MCB.

As Lucien concisely explained,with some units,you have 50 chances a second,to catch it out

 

[LeeH]

I take from the information in this thread that the correct OCPD will solve the tripling issues.

The transformer is fine and the C32 which had been on it has had its day. And the 2 times I tried reset it I was unlucky. However, it did go with quite a bang at the DB.

All basic tests on the circuit and transformer are OK now I know a low resistance is normal and actually required.

I'll just tip up with a D32 as the Zs complies.

Thanks all.

 

[Rpa07]

Let us know how it goes Leeh and thanks PEG for that info.

 

[LeeH]

Let us know how it goes Leeh and thanks PEG for that info.

With the D32 fitted you can turn it on and off like a mad man.

I'm surprised they never had issues with the C32.

Zs checked out when measured.