Well, yes. What am I missing?
I thought we were talking about ceeform plugs and sockets.
Anything requiring 10A or 5A protection should not be on a ceeform plug.
MCB for a compressor
- Thread starter JimboM
- Start date
I thought we were talking about ceeform plugs and sockets.
Anything requiring 10A or 5A protection should not be on a ceeform plug.
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Probably nothing - it's probably me that is doing the 'missing'. I did say that this was for my education!
We are.
Fair enough - but, as I said, the designer does not have any control over what gets plugged in.
However, as I said, I'm trying to learn, not argue. So are you saying that one designs such a circuit on the assumption that something 'inappropriate' (for a 16A plug) will not get plugged in? Are you also saying, by implication, that it is not acceptable to have two or more 16A ceeforms on the same circuit unless it is protected by a 16A OPD (thereby 'limiting' total load across all sockets to 16A)?
Kind Regards, John
So you did check the fault loop impedance at that socket before you went?
You do not know it works until you test it ot it operates to disconnect power when a fault develops.
All it is doing at the moment is allowing enough current to flow that the compressor starts. That is not its prime function.
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Indeed. Of course, a 32A MCB cannot really be 'tested' but one can at least measure the circuit characteristics to determine whether it ought to function correctly in the event of a fault.
It's not totally clear as to whether, having failed to find a C16, the OP installed a B32 or C32, although I suspect the latter, which could represent a problem...
As a sort-of cross-thread aside ... we recently discussed the newly published 'maximum Zs' requirements (to ensure proper OPD function) and the general view was that very few circuits were so marginal that the 5% reduction would be of any relevance - but, if it is a C32, the case we are discussing could be an exception to that. Even under 'current regulations', achieving a Zs less than the maximum permitted of about 0.72Ω might be difficult (theoretically maybe even impossible with TN-S) so that, even if the circuit could just 'scrape through' on current criteria, the 5% reduction in permitted maximum to about 0.68Ω might render it non-compliant come July. Although the OP's 'final final circuit' (from the garage CU) is apparently only 0.6m long, one imagines that there is probably an appreciable run of cable, contributing to the loop impedance, from that garage CU to the origin of the installation.
I'm not sure whether the OP intends the 32A MCB as a temporary or permanent measure but, if the latter, and if it's a C32, he really ought to get the loop impedance measured (indeed, he should really do that if it's a B32, or if he reverts to a C16 - just to be sure).
Kind Regards, John
Just to follow up, I happened to drive past another Screwfix today so as per my previous post I picked up a C16 and installed that. Everything still works fine and I'm hopefully no longer in danger of blowing up my garage.
Can I just clarify that the only reason I used the C32 was as a stop gap measure to help rule out the compressor itself as the problem. I haven't used the compressor other than to test it runs and would not have done so until the installation was correct.
Thanks to all that replied
Can I just clarify that the only reason I used the C32 was as a stop gap measure to help rule out the compressor itself as the problem. I haven't used the compressor other than to test it runs and would not have done so until the installation was correct.
Thanks to all that replied
That's better, and I'm glad that your compressor still works OK with the C16.
However, as I said, even with the C16 you ideally would have the earth fault loop impedance ("Zs", "EFLI") measured, to make sure that the circuit is safe with the C16 - the current maximum permitted Zs for a C16 is about 1.44Ω, due to reduce to about 1.37Ω later in the year. How long is the cable run from its origin in your house (CU or wherever) and the garage CU, and do you know what size of cable it is? Do you know what sort of earthing system your installation has (e.g. TN-S, or TN-C-S/"PME")?
Kind Regards, John
Although I don't have my copy of the Wiring Regulations on me at present, I am nearly sure that they expressly permit the use of a 20A device to protect multiple 16A commando sockets on a single circuit.
As part of my ongoing education, I'll attempt to find that! However, 20A is not much more than 16A - I was thinking more of a 32A OPD (assuming that the cable was up to 32A).
Kind Regards, John
314.3
It is all down to correct design.
It is all down to correct design.
Fair enough - but that's extremely vague.
As I'm always saying, it's really impossible to properly/correctly design a sockets circuit (that has more than one socket), since the designer has no control over what might be plugged into the sockets. If there are two or more 16A sockets, it's not impossible that a 16A load will be plugged into each of them, simultaneously.
Is your interpretation of the regs that, at least in some circumstances, one could have two or more 16A sockets on a circuit with a 32A OPD (assuming cable was OK for 32A)?
Kind Regards, John
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CHAPTER 43 PROTECTION AGAINST OVERCURRENT covers many pages and it would take some careful reading.
But the main point has not been answered. A motor overload and a MCB are very different beasts. If the compressor is fitted with a motor overload then in real terms it does not matter what MCB is fitted.
But if it does not have a motor overload the question is at 2.2kW (3HP) with a stall situation which will fail first the C16 MCB or the motor?
9.57 amp is not that far from 16 amp and under stall conditions it may cause the C16 MCB to trip. But should it not cause the C16 to trip then it will mean an expensive motor replacement.
Personally if it were my compressor I would not care one iota what the Zs reading was but I would care as to if the overload or motor went first.
A 3HP motor driving a saw bench and the user can see if the motor stalls and if it does do something to rectify the problem. However with a compressor he may be remote from the compressor and completely unaware it has stalled.
I have seen where the poor lapping of an exhaust valve on a compressor caused the cylinder to be filled with air at a pressure which caused the compressor to stall and rupture the supply fuses. This is a real problem not one which may in some unknown scenario may present its self.
We are talking about a compressor not some general motor and so the use of a motor overload is really important.
Fair enough - but that's because you are now talking about overload protection, rather than fault protection. To the likes of the 'EICR inspector', of course, the Zs would probably be the more important issue.
Kind Regards, John
Yes, but it is protected.
If that were a likely/frequent occurrence then it has not been designed properly.
I have no experience of commercial installations and am only applying logic and common sense and the relevant regulations.
What are these plugs and sockets for? - Appliances requiring more than 13A? Therefore ...
No. What would be the point of having 16A plugs/sockets if they can be plugged into 32A circuits.
More likely that they are used due to the increased IP rating of them.
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