Low Voltage transformer problem

[markymark2406]

I have looked at a lighting system in a resturant which consists of x8 12volt 50watt lamps off a transformer the problem is the Breaker will not stay in at the board, i suspect the transformer has gone down.

How do i meggar the transformer to tell if it is a problem and what results would i get if it is down or not?

 

[Spark123]

What type of transformer is it - wire wound or electronic?
What size and type of breaker is it?
Is the transformer double insulated?
Has it ever worked?
Megger is a brand name - I take it you mean Insulation Resistance test? Depends if there is a fault present and wether or not it is between a live conductor and earth, or between phase and neutral. If it is the latter then a normal resistance test might tell you more.

 

[securespark]

I would suspect it's toroidal & the high in-rush currents are popping the B rated breaker.

Just a guess, mind!

 

[Taylortwocities]

Do not do a 500v DC insulation test on electronic transformers. It will kill them.

 

[markymark2406]

its a wound transformer not a electronic one, the breaker is a 6amp type unknown.

So do i do an 250v insulation test between live neutral and live earth neutral earth?

What would i expect.

 

[Taylortwocities]

Come on. The breaker type is qwritten in it B6 is a B curve. C6 is a C curve.

Are you sure that you are conversant with IR testing.

What do you think you will get if you do an insulation test between live and neutral on this transformer????

If you insist on blaming the transformer then you could connect its live and neutral together and test between that and earth. Do it at 500V DC.

What else is on this circuit? Did this ever work, or is it a new problem.??

Does the circuit work OK with the transformer disconnected?

 

[markymark2406]

Yes the breaker stays in when the transformer is disconnected (this is done by a cliks connection) the problem hasn't always been there they did work.
In fact there is in total 4 transformers all feeding various downlighters one of the circuits on when the wall switch is switched it also takes another breaker out (but not all the time.

 

[Taylortwocities]

Umm, I'm somewhat confused but try this:

put back the suspect transformer and disconnect one of the other sets. Does the breaker stay in then.

Also, please advise the total load on the circuit.
You've got 4 wirewound transformers. how many 50W lamps on each?

if its 32 lamps @ 50W = 1600watts, way over the top for a 6 Amp breaker!

 

[securespark]

I think it may be accumulative. You may get away with (say) 3 tx's in circuit but add another & the B6 trips on start-up.

 

[Taylortwocities]

SS, why didn't you think of that earlier

.... the high in-rush currents are popping the B rated breaker.

Oh, but you did

 

[ericmark]

Come on. The breaker type is qwritten in it B6 is a B curve. C6 is a C curve.

Are you sure that you are conversant with IR testing.

What do you think you will get if you do an insulation test between live and neutral on this transformer????

If you insist on blaming the transformer then you could connect its live and neutral together and test between that and earth. Do it at 500V DC.

What else is on this circuit? Did this ever work, or is it a new problem.??

Does the circuit work OK with the transformer disconnected?

Maybe you can add an arrow to where the B, C, or D is written?

I have tried to test transformers in the past and even when there is no smell, the IR test is OK and the resistance on each winding seem to match with others of same type they have still been faulty. A few shorted turns is not likely to show up and one has in the end decide is it worth the time to play with it?
There comes a time when you have to just say it is uneconomical to test just renew it.

However when you say adjoining MCB's also trip it does point to them all being on the edge. You say a test with no secondary load it will hold. And if pure thermal trip like one shown then first thing in morning when cold it may hold and only show as faulty once everything has warmed up.

An ammeter would be handy although you can still get the old MCB
See here they are hard to get and likely non compliant and some consideration as how to proceed would be needed.

 

[bernardgreen]

A few shorted turns is not likely to show up and one has in the end decide is it worth the time to play with it?

It is possible to check for shorted turns by measuring primary current when there is no load on the secondary. A perfect transformer would take no current in that situation. The perfect transformer does not exist so the normal no load current for the transformer being tested has to be known by measuring it for a known good transformer of the same type.

Shorted turns act like a loaded secondary and take power and increase the primary current.

 

[bernardgreen]

I think it may be accumulative. You may get away with (say) 3 tx's in circuit but add another & the B6 trips on start-up.

If it is accumulative then the tripiping on in-rush problem can be eased by sequencing the energising of the transformers so they do no all "in-rush" at the same time. At switch on power goes directly to the first transformer. The ouput of this powers it lamps and the coil of a contactor the contacts of which apply the switched supply to the second transformer. The second transformer energises a contactor to apply power to the third transformer and so on.