Lighting transformer tripping MCBs

[JimM]

Hi guys

Further to a previous post I've purchased the transformer in the link below for my garden lights,

as soon as I turn it on the MCB/RCD trips on my CU :( Originally I thought the unit was faulty so I got a replacement sent but both units have the same problem.

I'm running the transformer on a 30A ring main but it is through a 13A plug is this likely to be the cause of the problem? If not any suggestions of basics tests I could do to see if it the transformer itself that is at fault (obviously the unit is sealed so I can't poke around inside)

Many Thanks

Jim

[Bob Minchin]

>> However as soon as I turn it on the MCB/RCD trips on my CU :( Originally

Quite likely to be the input filter. Disconnect the earth lead. It won't be doing anything useful but make sure one side of the output is earthed.

Bob

[JimM]

Hi Bob

There's no earth lead to disconnect, just live and neutral on the mains side and presumably 12v and 0v on the output.

Not sure if it makes a difference but when testing I tried with the lights connected and not connected both with same results

Thanks

Jim

[Colin Stamp]

>

The shape suggests it probably has a toroidal transformer in it. They can have huge switch-on surges that trip MCBs of a much higher rating than you would expect. You can get MCBs that are designed to cope with bigger surges, but fiddling with the CU just to accommodate one awkward transformer is a bit silly really. I generally assume that big toroids aren't really compatible with MCBs.

See if you can find either a laminated or electronic transformer.

Cheers,

Colin.

[Andy Dingley]

Big inductive load. Try a C curve MCB, or even a different B curve MCB (some are just twitchy after years of service).

[Tabby]

Yes, its a 30A mcb though. Normally such things are used on 5 & 6A circuits.

Possibles: faulty unit from same batch - most likely socket wired wrong, n/e confused

NT

[The Natural Philosopher]

MOSt decent transformers have cold start thermistors to limit switch on surge. Kinder to the bulbs as well. Think yu can buy those separately.

[John Rumm]

>> However as soon as I turn it on the MCB/RCD trips on my CU :( Originally Well which, the MCB or the RCD or both (or have you got a RCBO)?

30A would suggest a fuse, rather than a 32A MCB...

not a chance....

bit more info needed.

[Colin Stamp]

Never underestimate the capacity of a toroid to trip an MCB. In the worst-case, a cheap one will have virtually no inductance whatsoever, just a few ohms of copper straight between L and N. They trigger the magnetic (short circuit) bit of the MCB rather than the current overload bit, so the actual MCB rating is less important.

Could be but, if it *is* a toroid, then I still wouldn't be surprised if they don't "all do that sir".

Surely the OP would have noticed that anything plugged into that socket would trip the RCD?

Cheers,

Colin.

[Tabby]

They need to be suitable for purpose, ie run ok on a 6A mcb.

Even badly behaved toroidals have ohmic primary resistance. A 100w transformer with 10% regulation will surge upto 10x rated current when inductance is temporarily zero. If this is the problem, the OP would notice that sometimes it trips, sometimes it doesnt.

At the end of the day, if it wont work on a standard 30A breaker, its little use to anyone, and either faulty or unfit for purpose. I'd reject the goods as faulty, and refuse yet another one from the same batch.

Yes - it just might be a forgotten socket that hasnt been used in years.

NT

[Dave Plowman (News)]

However as soon as I turn it on the MCB/RCD trips on my CU :(

By its shape, it's a real transformer. The inrush current on these can trip an MCB as it can be very high for an instant. Do the instructions cover this - like say stating a type C MCB is needed?

It shouldn't trip an RCD - that would suggest it's leaking to ground on one leg. So faulty.

[The Natural Philosopher]

Mine dont have a ground.

If its tripping an RCD its a sign of some high residual earth currents (RCD on the edge anyway) plus a rather high inrush current.

[Dave Plowman (News)]

Can you explain how an RCD can trip on something like this with no ground connection? Ie, how can you get an imbalance on the current flow between line and neutral?

[The Natural Philosopher]

Usually because there is a neutral to earth short somewhere in the building or a high enough impedance in the live such that switch on causes a mometary voltage drop in the live that is enough to trigger, via all the capacitors in all the RFI devices between ground and live, the RCD.

[John Rumm]

Its a known problem with most RCDs when they are "pre-sensitised" (i.e. the nominal circuit leakage has used most of their trip current budget, so they are very close to tripping).

The inrush surge can generate enough distortion of the supply waveform to inject lots of harmonic noise into the supply. This is coupled to the system earth via all the mains input filter in your appliances, and that can briefly give you enough extra leakage. Note some older RCDs are sensitive to the surge in their own right as well - even without the help of external appliances. The same trip scenario can occur from externally generated spikes etc as well.

[Andy Wade]

I wouldn't expect a single 100 VA transformer to trip a 32 A Type B MCB. The instantaneous inrush current cannot possibly exceed the peak mains voltage (about 340 V) divided by the resistance of the primary winding. A 100 VA transformer will have a primary winding resistance of around

10 ohms (give or take a bit), so the worst-case current spike is about the same as the continuous current-carrying ability of the MCB. To trip the B32 MCB on the magnetic part of its characteristic needs a surge current of at least 3*In, i.e. 96 amps. A transformer that could do that would have to have a primary resistance of less than 3.5 ohms, which takes you into the territory of transformers much larger than this one. With a 5 or 6 A MCB it might be a different matter.

If an MCB-trip is occurring the transformer must be faulty - primary shorted turns - or incorrectly rated - 110 V primary? - or connected with primary and secondary transposed!

Or incorrectly connected - primary between L & E - stranger things have happened...

[Mike Tomlinson]

In article , JimM writes

Is that with it submerged or out of the water?

You are fitting the mains plug to the correct wire, yes? :o)

[Colin Stamp]

Absolutely. What it needs to be and what it is are two different things though :o)

I don't think that quite follows. 10X rated current might be what you'd get if you shorted the secondary, but that's not the same thing as peak mains voltage divided by the primary DC resistance. Having said that, I admit it is still probably a bit of a push for a 100VA transformer to trip a 30A MCB.

Yep. Regardless of what it's tripping and why it's tripping it, getting a new one of a different type is probably the only way out.

Cheers,

Colin.

[Andy Wade]

You're quite right - that doesn't follow. If the transformer prim'y and sec'y have roughly equal copper areas then half of the impedance will be attributable to each winding. So in Tabby's example the worst-case surge will be 20x rated (RMS) current, times 1.4 to allow for instantaneous peak values.

As I tried to say earlier that simply won't happen, unless there's something wrong with it, or unless you've connected the secondary to the mains (which does tend to make things go with a bang (DAMHIKT)).

[Colin Stamp]

Could be. I've come across plenty of instances of toroidals taking out MCBs though. Probably smaller MCBs, though still pretty impressive.

My own main experience with them was long long ago, and the transformers were bigger - 750VA auto-transformers. When I started testing one, I just put a 13A plug on it and plugged into a the socket ring in the company "lab". After a couple of switch-ons, the lab MCB on that ring didn't just trip, it broke! It wouldn't stay latched on after that and I had to replace it. The upstream building breaker feeding the whole lab CU also tripped. I had to resume testing in the distribution panel cupboard so I could keep resetting the breaker. Of course this was a rush job and I had to find a way of making them MCB-friendly...

I can't remember the ratings of the MCBs, but I remember boggling at the time. The DC resistance between the 0 and 240V taps was around 1 Ohm IIRC.

Cheers,

Colin.