MCB and a Transformer

[Shadowfax]

A short while ago I exchanged all the rewirable fuses in my Wylex CU for plug-in MCBs. I discovered this week that the transformer that reduces the power for our treadmill has a high enough inrush current to trip the 20 amp MCB. It is on a very small circuit with a couple of fridges and the treadmill on it - nothing else.
I thought I would try a 32 amp MCB but the result is the same. The MCB trips sometimes, as soon as the transformer is switched on but not every time.
I tried the transformer on a different circuit and the same thing occurs.
There is no fault with the treadmill, it is the transformer that trips the MCB with nothing plugged into it.
It is a 2KVa 115V thing similar to a site transformer.
I guess what I really need is a Type C MCB but Wylex do not make them in the plug-in type.
Any ideas?
For now I have replaced the 20 amp fuse and all is well.
I had forgotten til now just how much of an inrush current you can get with a transformer.
Cheers.

SF

 

[Smudger]

Sorry, Shadowfax, if this is way off-topic, but as you seem to know what you are talking about, can you shed light on a computer problem which may be related?
Recently I was given an iMac by my son who lives in the States. It is dual voltage. When I plugged it in it tripped my main MCB, so I took it to an Apple reseller who checked it over and said it was OK. When I tried it, got the same result. I tried it in our house in France - OK if I disconnected the earth, but as soon as I connected it to a printer it tripped the main MCB. This was without the computer being turned on.
Any ideas most gratefully listened to.

 

[RogerS]

Smudger..I'm guessing here but it sounds as if you have a leaky power supply and that it is leaking to earth. I'm assuming that at the places where you have experienced the problem, your consumer unit is fitted with an RCD. This expects to see the same current going out the neutral as is coming in from the live. If they differ by a set amount (usually 30mA) then the RCD will trip. The Apple store may not have been fitted with one of these devices which is why they said the iMac was OK. Or they may have been using an isolating transformer on the workbench.

Which leads on to the easiest but least elegant solution namely to buy an isolating transformer and plug the iMac into that. Or dig deep and buy a new PSU for the iMac. I'm assuming you have switched the PSU setting on the iMac to 240v (sorry, I'll get me coat).

Shadowfax - you'll need something like this in series with the mains feed to your transformer.

 

[Shadowfax]

Smudger
Are you talking about a miniature circuit breaker (MCB) or a Residual Current Device (RCD)? When you say Main MCB I guess you are referring to the device that protects all our circuits rather than just one.
If I am right it sounds like you have an earth fault on your computer or its power supply, at least. The RCD is looking for a current imbalance and is tripping because it is finding one. An MCB will trip if the current it is monitoring is exceeded.
I think you need a new power supply for your computer.
Cheers.

SF

 

[Smudger]

Thanks, Roger, and Shadow, that suddenly makes it clear, especially about the workbench test clearing it. Also how isolating the earth hid the problem.

It's a lampstand iMac, which is a lovely thing in my eyes, so it's probably worth a PSU (about £100 I think) - especially as the computer was free! Sometimes bringing kids up turns out to have been worthwhile :wink:

 

[Shadowfax]

Thank you Roger.
That looks like just what I need. I ordered one straight away.
I don't like to be beaten by a simple bit of kit like a trannie!
Update to follow.

SF

 

[Shadowfax]

Just as an up-date. I got the current limiter from Maplins.
Set it up last night in series with the mains input to the transformer.
Switched on and nothing happened - the transformer worked and no MCB's threw their arms up in horror!
However, I switched of to see what hapened if I repeated the procedure.
Pop! Out went the MCB. The current limiter is a thermister and had heated up so was letting more current through - too much for the MCB.
At that point I thought it might be best to just put the fuse back in place of the MCB as that situation gives no trouble at all.
The conclusion I have drawn is that the current limiter probably needs a circuit of some sort associated with it that would get over the re-starting problem. It also occurs to me that the limiter is going to get pretty hot in use and therefore will need some means of keeping it cool or, at least, it will need to have some air flowing around it.
A frustrating little problem but hardly the end of the world.
Thanks for your idea though, Roger. It worked but might need some tinkering.
Cheers.

SF

 

[johnemtee]

Get rid of the treadmill!
Plane some timber instead. :twisted:

 

[Electricsmog]

What size is this current limiter? And how have you connected it? I cannot see any reason for a maximum 9A load to trip a 32A MCB. Your treadmill will have some sort of soft-start, or you will be hurled accross the room at start-up.

Does the transformer trip the circuit when not connected to the treadmill?


***********

Sorry just re-read your original posting. It sounds like the transformer is breaking down internally, try connecting a multimeter to it in series with no load connected to the transformer there should be minimal current drawn, any variation suggests shorted turns.

 

[Shadowfax]

Electricsmog, Hi.
Sorry I have only just seen your post.
Forget the treadmill. It is not even plugged into the transformer when this happens.
The current limiter was an 18 amp one and was just wired in series with the input to the transformer - purely as an experiment really.
Worked perfectly but I 'm not sure if that is OK because it will obviously warm up so it would need to cool again before it would work again.

So the answer to your second question is "yes".
Cheers.

SF

 

[Woody Alan]

OK this might be bit off the wall here. You could try putting a 100w mains light bulb in series with the transformer with a light switch in parallel with the light bulb. The sequence would be turn on Transformer whilst light switch is off i.e. bulb is on. At this point both the Tfmr and bulb will be in circuit at reduced power. Then operate the light switch to short out the bulb. If trying this you need to make sure the switch is of a high enough rating. This should at least prevent the start up surge. I must say though I am not sure this should be happening at all it's normally only toroidal Tfmr's that give this trouble. It's all related to a shift of current and voltage going out of phase because of the induction, I guess . :roll:

Or just put the switch across your current limiter device and when tfmr is on, short out the limiter and when you switch off unshort the limiter and then turn off tfmr, if it is giving problems on switch off as well.

Just another thought your fridge and freezer aren't really ancient are they running really hot around the compressor, Is it possible it is they that are pushing up your cct loading to cause this?

Alan

 

[Shadowfax]

Alan
I actually thought of doing what you suggest but I got fed up with the process and just replaced the 20 amp fuse. I think you are right about the out of phase thing because it does not happen every single time.
I have been testing this on a different circuit, in my workshop, so the fridge does not play a part nor does the treadmill. It is just the transformer that trips the MCB. With the fuse all is well.
The by-pass switch would certainly get over the heat business. Maybe a push-to-contact switch that does not latch would be the safest device to use. If I can find one!

I might not be able to reply for a day or two. Visitors this weekend!
Many thanks to all you guys who have been mulling this over.


SF