Running 2 transformers off 1 switch: tripping

[zygo]

I have just installed a low voltage halogen lighting system in my converted office - the type that uses flexible cable tracking suspended from the ceiling. From the twin track hang five 12v 20W MR16s (in parallel). The track is powered by a heavy toroidal transformer. The supply is taken directly from a secondary 2-way CU serving only this room.

In fact I have two complete track systems (i.e. five on the left and five on the right) each sytem with a separate transformer. I have linked the two transformers together and run the power from one light switch to them both.

The problem: when both transformers are connected, the 6A MCB trips (not every time but far too frequently). However, when only one transformer is connected (left or right) it's fine.

The only aspect of the circuit I am not sure about is that both transformers run off the same switch. If these were two 60W bulbs in two bulbholders, I would expect no problem. But is this an inappropriate way to connect the transformers? Should I run each from a separate switch instead?

Many thanks

[sparx]

Hi Zygo,
I think the problem is the 2 toroidal transformers inrush current, as I think you may have guessed.
Although the steady load is only around 200Watts plus the magnetising current for the 2 transformers the problem is similar to motor starting load. I am assuming a type B 6A circuit breaker is fitted as standard?
If you can change to a 2 gang switch and operate one bank at a time the problem will go away, however if wiring concealed and finished then the only solution will be to change the Circuit breaker to a type C 6 Amp which will take a greater inrush 'let through' load however it will reduce the total circuit impedance Zs somewhat. EG 6A type B=30A for 0.1s, type C = 60A for 0.1S. Should not be a problem as difference of Zs is
Type B = 7.67Ohms, Type C = 3.83Ohms which is still quite a lot for a small lighting circuit.
I assume this means something to you as from your post you seem to have a reasonable idea of basics, very difficult to judge on here how much info is too much info.....
regards SPARX

[zygo]

Thanks, Sparx.

I am able to follow the logic (though maybe not the maths!) and it is inkeeping with what I suspected. My original design was to run two circuits on two switches but I decided to combine them and use the second switch for something else. so it was not too difficult to go back to the original plan and, like you say, everything is fine (even if I switch both on simultaneously, within 0.1 sec of each other!)

Had I had a spare C-type MCB, I would have tried it after reading your solution but unfortunately the C-types that I have available, are for the main CU which is a Wylex brand with screw-in MCBs, whilst the secondary one (Crabtree) has the push in type; so they are not interchangeable. Shame; though I could always get a push-in C-type as the only other thing on the same MCB circuit is a 60W lamp so the total would be well within the lower 3.830 Ohm rating.

A very helpful reply. Thanks again.
Z

[sparx]

Hi Zygo, glad your sorted, it's nice to get some feedback.
I keep a box of obscure CB's such as Sq. D & Crabtree plug-ins just in case as some are difficult to get now,
regards SPARX