MCB for a compressor

[JimboM]

Hi all

I've put in a new radial circuit for my 3hp compressor but every time I fire it up the MCB trips immediately. I've tried with a 16A and 20A MCB but get the same result. The compressor was working fine at my old house so I'm assuming it's down to the install and not the compressor.

The circuit is a 60cm run of 6mm cable from the CU and terminates at a 16A socket. The MCB's I've tried are both Type B and I suspect that may be the issue.

If I try a Type C MCB is there any issue with using a 32A one or should I go back down to 16A ? My local stockist hasn't got any 20A ones !

Cheers

 

[Adam_151]

is there any issue with using a 32A one

terminates at a 16A socket

 

[ban-all-sheds]

I've tried with a 16A and 20A MCB but get the same result. The compressor was working fine at my old house so I'm assuming it's down to the install and not the compressor.

Did the old house use fuses or MCBs?

If I try a Type C MCB is there any issue with using a 32A one or should I go back down to 16A ? My local stockist hasn't got any 20A ones !

Socket rating rules out 32A.

Have you checked that your Zs will allow a Type C?


FYI:

If the startup surge is tripping a B20, it could easily be causing interference to your neighbours' supplies.

If you've not applied for permission from the DNO to use the compressor then you will be in breach of your agreement, and if your neighbours complain the DNO will make you stop using it.

One way or another.



This is about a welder, but the appliance is irrelevant - the problem (if there is one) is what counts: //www.diynot.com/diy/threads/welding-socket-needed-for-17th-edition-board.417054/

 

[ericmark]

I read the word compressor and remember problems of the past. One of the problems is the de-loading and there are many methods used.

The time taken to start is also a problem and volt drop can extend the time to a point where resistance builds up before it reaches running speed.

In other words it's unlike nearly every other electric motor driven device the only other one similar is a pump and of course the compressor is pumping air.

There are also numerous cures.

If the magnet part of the trip is rated too low then it will hardly try to start. So if we look at the magnetic part we have.
B16 = 80A = 2.875Ω
B20 = 100A = 2.3Ω
C16 + B32 = 160A = 1.4375Ω
C20 + B40 = 200A = 1.15Ω
D16 + C32 = 320A = 0.71875Ω
D20 + C40 = 400A Ω = 0.575Ω
All old figures not new amendment.

But as you can see a C16 and B32 are the same as far as start current. Where the loop impedance is not good enough then you have to look at an alternative starting method. Today pumps and compressors are more and more going to using three phase motors and inverter drives both my fridge/freezer and freezer use that method.

I have seen where a compressor will start from empty but not once there is any air in the receiver this is completely different to just will not start. At 9.8A run with no losses with losses will be over 10A draw there is very little hope of it starting with a B16.

I would expect 100A start load even without turning the compressor i.e. drive belts removed. I expect it's not 600 mm of cable I will guess you meant 60 meters if that's the case then at 100A it will be a 85 volt drop clearly that is a non starter the only way would be an inverter and changing to a 3 phase motor.

Cheaper option may be have compressor at house and an air line to where it is being used?

 

[JimboM]

Hi thanks for the replies. As it turned out my local Screwfix didn't have any 16A type C's so I've temporarily fitted a 32A and it works fine. I'll get a 16A when they have them back in stock and see how I get on with that.

For clarification I did mean 600mm cable run, the garage has it's own CU and I've mounted the socket next to it.

 

[RF Lighting]

Unacceptable. You were told on here you must not install a 32A device. You ignored that and went ahead and fitted one anyway. Why bother wasting our time?

 

[ericmark]

Unacceptable.

If the motor is fitted with an overload I would be looking at 433.2.2 carefully before condemning.

I have seen many a 110 volt transformer with a 12A thermal overload on incomer and 13A fuse as only current limiting devices. So the current from a 16A socket could be as high as 50A before the 12A incoming current is exceeded.

This has caused leads to melt but not the plug or socket. Clearly although rated 16A it can carry much more than 16A.

In a commercial situation you have no idea what may get plugged into the socket but in a domestic as related to here it is unlikely anyone will plug in something other than the compressor so as a temporary measure then as long as there is a overload on the compressor although non compliant it is unlikely to cause a problem.

However if there is no overload on the compressor then it's a completely different situation. All it needs is for a de-loading valve to stick and you could burn out the motor.

 

[JohnW2]

is there any issue with using a 32A one

terminates at a 16A socket

If I try a Type C MCB is there any issue with using a 32A one or should I go back down to 16A ? My local stockist hasn't got any 20A ones !

Socket rating rules out 32A.

I think I need some education here, probably mainly because I have limited experience of sockets circuits which do not use BS1363 sockets. Do these statements relate to an attempt to 'protect a socket+plug' with an OPD and/or a (not very effective) attempt to use an OPD to 'limit' the current drawn by a load (so as to limit what goes through the plug/socket), or what? Do I take it that the oft-quoted concept of the OPD being there "only to protect the cable" has gone out of the window?

Kind Regards, John

 

[Adam_151]

Unlike a 13A socket john, the plugs for these are unfused, So there is nothing to stop a 32A load being connected to a 16A socket.

While you could say that the 32A load should not have a 16Aamp plug on, there are out there such things as splitters and converters. A converter lead to allow a 32A plug to be plugged into a 16A socket is seen as an acceptable piece of kit, whereas the other way round would be unacceptable unless it also included over-current protection

 

[JohnW2]

Unlike a 13A socket john, the plugs for these are unfused, So there is nothing to stop a 32A load being connected to a 16A socket.

I obviously realise that, hence my initial comment about my limited experience of sockets circuits not using BS1363 sockets (hence fused plugs). However, as I said, using an OPD as a means of restricting the size of a load is not a usual concept, and not even one which would be very effective - a C16 (which is probably what the OP should be using) would, for example, allow considerably more than 16A to flow for an appreciable period of time.

As I said, given that (in the case under discussion), the cable is more than adequate for 32A (hence for a 32A load), it sounds as if you are talking about attempting to use an OPD just to 'protect a socket+plug' - which is not what one usually expects an OPD to be used for. As I said, all part of my education! Furthermore, as I wrote yesterday in relation to other connectors, without knowing details of the 'over-current behaviour' of the plug and socket in question, I wouldn't even know whether a 16A MCB (of any Type) would be adequate to 'protect' a "16A" plug and socket.

Kind Regards, John

 

[ericmark]

I have seen many times where a 16A socket has a over current device over the 16A rating and also seen where it has caused problems.

It was common for two 16A sockets to be protected with a 20A MCB. As a result we had some vacuum cleaners with three 7.5A motors which would run without a problem off a 16A socket with 20A MCB.

However on one site run by Nuclear Electric we were not allowed to have 20A MCB on two 16A sockets we had a separate 16A MCB for every socket. But the cleaners would arrive on site with a 16A single phase plug.

We would have to rewire the cleaner and we would fit a 4 core cable with a three phase 16A plug which would in turn cause problems when it arrived at another site.

It is rare on a building site to find 32A or 16A three phase outlets on the 110 volt system so it is common to find well over 16A being drawn from a 16A outlet. Other than the colour of the plastic and locating lump there is no difference between 110 volt and 230 volt plugs and sockets so for experience I know there is no problem drawing over 16A with these sockets.

That of course does not make it right and one would assume in the old premises there was also problems with the size of MCB to socket size.

I would want to see an overload

the type shown will replace the MCB for may applications but the

type connected to a contactor will not but these are set more finely to the motor current than any MCB. In early days they were magentic devices with a dash pot to slow them down to allow the motor to start. Today most are thermal but although allowing a high start current they will trip should the motor stall much quicker than a MCB.

With a compressor a fault with the de-load can easy cause a stall and without one of these devices it's easy to burn out a motor.

 

[EFLImpudence]

it sounds as if you are talking about attempting to use an OPD just to 'protect a socket+plug' - which is not what one usually expects an OPD to be used for.

I doubt that plug/socket require protection.

They are to stop the connection of machines to an inappropriate circuit.

 

[JohnW2]

it sounds as if you are talking about attempting to use an OPD just to 'protect a socket+plug' - which is not what one usually expects an OPD to be used for.

I doubt that plug/socket require protection.

So do I (and, as I've said, even if they did, I'm not sure what would be required to protect' them) - but ....

They are to stop the connection of machines to an inappropriate circuit.

That is clearly what everyone is implying, but what does it actually mean? If the supply is capable of supplying 32A, the cable is more than capable of carrying 32A and the circuit has a 32A OPD, then what is 'inappropriate' about that circuit supplying a 32A load, other than the presence of the "16A" plug/socket (which, as you say, probably doesn't need 'protection')??

Kind Regards, John

 

[EFLImpudence]

The machine and associated flex may require 16A protection and has to rely on the CPD.

 

[JohnW2]

The machine and associated flex may require 16A protection and has to rely on the CPD.

True, but the machine and/or associated flex might requite 10A, or 5A, or whatever protection, mightn't it? - in which case, by extrapolation, you'd presumably say that it was 'inappropriate' even if the OPD were 16A, wouldn't you?

It seems to be the standard problem as with any sockets circuit - the designer has no control over what might get plugged into it, and therefore can't really design on the basis of that 'unknown'.

Kind Regards, John