LV Lighting Tripping RCD

[Charles Middleton]

Hi,

I've just fitted some Low Voltage lighting in my bathroom and it seems to randomly trip the RCD.

The set up is as follows:

- Twin and earth from original fixture now wired into transformers input.

- Transformer is from Newey and Eyre and is a 4 x 20-50VA output toradial model.

- Twin and earth running from transformer to each fixture (all runs are the same length).

- Each LV fixture has a 35w halogen on it.

I've tested the inputs and the outputs of the transformer and they are all fine. I've also tested each fixture with a lamp independently. One of the things I found is that the switch controlling the lights (which is a standard light switch located on landing - not pull cord) seemed to be very, how can I describe it - creaky(?) and I got the impression that it wasn't making a clean connection. So I've changed that and now the RCD trips less when I use the system. In fact in an hour of random on/offs last night it didn't trip at all.

The problem is I've lost confidence in the installation now as I certainly wouldn't want to be caught short in the middle of the night with no bathroom lighting! So is there anything else I can check, or from what was read above anything I have done wrong?

Thanks in advance for any help,

CM.

[stevecope]

[Charles Middleton]

Perhaps I have the terminology wrong - as I understood it the breaker in the consumer unit was an RCD??

What I should have said was the the lighting trips the circuit breaker in the consumer unit.

CM.

[Harry Bloomfield]

On 01/02/2004 Charles Middleton opined:-

To put it simply, an RCD is a device intended to protect you from receiving a shock. The RCD can be identified by the fact that there will be a button on the front marked 'test'.

A circuit breaker is called an MCB. For your lighting circuit this will probably be rated at 6amps. Likely the problem will be the large surge of current which flows as the transformer is switched on. This is further confirmed by the fact that your faulty switch caused the MCB to trip. The poor switching characteristics will have caused several surges of high current to flow, each time you used it. Whilst this is not my area of expertise, I believe the usually suggested solution is to change the MCB for one which provides some surge resistant characteristics...

The MCB you have fitted will likely be a 6amp B type, change that for a

6amp C type. Supply the model details from the actual MCB which is tripping, someone will be able to provide more help and perhaps a replacement type number.

[The Natural Philosopher]

Odds are that you had a marginal installation with some erath/neutral leakage anyway, and this rather uinusal load has pushed the lot over the edge.

I had this problem, and replaced teh 30mA breaker with a 100mA one. Problem solved.

I SHOULD put a split load unit in to get 30mA back on downstairs sockets, but can't be arsed...

[The Natural Philosopher]

Which one? The lighting circuit only, or the whole board?

[Charles Middleton]

The lighting circuit.

I'll also supply the details of the board as per message from Harry when I get back from a trip later this week.

Thanks all for your help.

CM.

[The Natural Philosopher]

Oh, fit a slo blo MCB.

Surge currents on toroids are enormous.

Better ones come with a slow start thermistor.

[Charles Middleton]

In regard to this Type-C MCB, I imagine that this allows a period of time where high current is passing through it before it trips. Are there any adverse effects from installing type of breaker? Would it trip before such an overload would perhaps overheat cables or cause any damage?

Thanks,

CM.

[Christian McArdle]

MCBs trip on two different mechanisms. There is a short circuit mechanism that trips immediately if a certain level of current is drawn. There is also an overload mechanism that trips if a lower amount of current is drawn, but over a longer period. Type B and C breakers have the same thermal trip characteristics, but require different currents for the instantaneous trip. For comparison, a fuse only has the equivalent of the thermal protection.

Both a Type B and Type C breaker will protect the cable itself adequetely, as the cable only really needs the thermal trip for protection. However, if there is a requirement to disconnect within a certain time for protection against indirect contact or fire in attached appliances, then calculations may need to be done to ensure that the cable isn't too long. If it is too long, then the fault impedence may be so high that the current required to immediately trip isn't reached even in a dead short. Unless the lighting circuit is huge, this is unlikely to happen with a Type C breaker, as the cable used for lighting circuits is usually well uprated over that required and it is easy to get the required short circuit/earth fault current. This is less true of power circuits, which don't normally overrate cables by the same degree.

As lighting failure during house fires is a common and measurable cause of injury and death, this can offset any additional risk due to the slower tripping in marginal cases when using Type C breakers.

Christian.

[ng_abuse-subscribe]

Harry Bloomfield retched LV Lighting Tripping RCD onto my recliner:

Could a simpleton put it any other way?

[Martin Angove]

The other option - if it is a standard light switch rather than something like a pull-cord - is to buy a "soft start" dimmer switch. It costs a lot more than a new MCB (maybe 18 quid versus 8 quid) but can look quite sexy, and avoids the need to open up the consumer unit if you're a bit worried about that kind of thing. We had a similar problem in our kitchen where we had 175WW (5*35W) on one transformer. Fitting the dimmer did the trick nicely, and as the kitchen was also the main entrance to the house, always impressed visitors :-)

Hwyl!

M.

[Lurch]

If the transformers dimmable that is...

SJW A.C.S. Ltd.

[Martin Angove]

The OP described a toroidal transformer. This not electronic and hence dimmable given a suitable dimmer switch.

(Yes, I know that many/most modern "electronic" transformers are dimmable too, but these often have soft or zero-crossing start built-in and are less likely to cause the symptoms described. Toroidals are particularly bad in this respect, though they are gorgeous electrically.)

Hwyl!

M.