MCB trip nuisance

[Observer]

We have a Wylex CU installed in a small retail unit. One of the ways has a 10A type B MCB that switches the window lighting circuit. The load is

- 300W toroidal transformer supplying ELV to a track light with 6 x 50W fittings

- 6 x small 50W toroidal transformers supplying ceiling mounted ELV downlights with 50W lamp.

Not sure what the current rating of the above is but it's surely nowhere near 10A.

Switching the lights on in the morning (but only in the morning) keeps tripping the MCB. Once the MCB has been reset, the lights can then be switched normally without any problem.

I understand that replacing MCB with a type C can alleviate this type of problem but I understand that Wyle don't make them.

Any suggestions on action I can take to resolve this or further diagnosis?

 

[Adam_151]

I understand that replacing MCB with a type C can alleviate this type of problem but I understand that Wyle don't make them.

http://www.screwfix.com/app/sfd/cat/pro.jsp?cId=A331832&ts=47082&id=34275

[reminder]ELFI test, certification, competant person as per EAWR, etc....[/reminder]

 

[Steve]

When you say "Wylex CU", you're not referring to a Wylex standard fuse box, are you? Type C are NOT available for them, but for new non-plugin Wylex boards, they are available.

 

[Observer]

When you say "Wylex CU", you're not referring to a Wylex standard fuse box, are you? Type C are NOT available for them, but for new non-plugin Wylex boards, they are available.

No - it's a new (1 year) split load CU. The local branch of TLC told me Wylex don't make type C MCBs and I can't see any in the TLC catalogue but, as Adam's link above, they do exist. Don't understand why TLC say they don't, unless the Screwfix ones aren't compatible.

Anyway, would changing to type C solve the problem? I've got loads of ELV downlighters at home (all on electronic transformers) and never have a problem. Is that because electronic transformers are 'soft start' and toroidal transformers aren't?

 

[plugwash]

it probablly will, real transformers driving downlights tend to have much higher switch on currents than those "electronic transformers".

but don't change to a C10 without checking the figures for the installation are suitable and note the comments from others about EAW.

 

[securespark]

The EFLI figures for type C are half that of the type B's.

You MUST ensure that the EFLI at the destination of this circuit is at or below this figure before you change the breaker to a C type.

 

[bernardgreen]

The first cycle in rush current on a torodial transformer can be 20 times the continuous rated current at maximum load.

So use separate switches for each transformer if possible

Having been faced with this problem ( 7 big trannies ) the solution was to have 6 contactors fitted.

The first tranny was direct to the switched supply. The second was to the switched supply via a contactor the 12 volt coil of which was fed from the first traany. Third tranny via contactor supplied by second tranny and so on.

The other solution were the trannies are not close together would be to use six delayed ON contactors with different delays. ( such as Merlin Gerin RTA series )