Garage RCBO tripping large machinery

[jmac80]

Hello all
Wonder if anyone can shed any light on this situation.
The problem is i have a Charnwood planer thicknesser the book say's it's: Motor ( induction) 3000w (4hp) 240v & requires a 16amp electric supply.

This machine is in my attached garage that has it's own hager consumer unit, The CU is fed from the house CU on a normal 40a mcb (about 10m) via a 10mm 2c+e. There is no other load from this garage CU, Just the commando socket at the moment..

The problem is.. Charnwood's website states a 16a connection is needed but the manual says 20a..
So.... The commando socket about 3m from garage CU is fed with a 4mm radial cable.
I tried the machine on a 16a rcbo type B , not a chance and tripping every time i start the planer, fine i guess...
Then tried a 20a rcbo Type B and it trips half the time on startup.
Now just tried it on a 32a rcbo type B and it didn't trip once...
Thing is this machine should really run on a 20a rcbo without tripping, not even as if it's cold at the moment.
Any ideas on this, could a 20a rcbo type C solve this?

Thanks

 

[phatboy]

You will be wanting a C or D curve breaker I reckon. B curve will not allow for the inrush current at startup.

 

[Iggifer]

I would try a 20a type C, see if that cures the inrush problem. I would be hesitant to go to a D type unless you absolutely have to

 

[Jackrae]

The supply breaker is there to protect the cable feeding the socket, not the machine attached to the socket. If you are concerned about overload protection of the machine then you should fit the appropriate device to the machine itself.

 

[ericmark]

B = 5x, C = 10x and D = 20x so a 32A B type will allow 160A start up load. Also a 16A C type will also allow 160A.
A 20A B type would be 100A which you say is not good enough.
So using a C16 seems the right way to go.

What we have to remember is 230v / 160A = 1.44Ω so the earth loop impedance must be better than 1.44Ω if using either a C16 or B32 unless a RCD is used in which case the PSCC needs to exceed 160A. It is very easy to just say fit a C16 but without either the prospective short circuit current or the loop impedance one does not really know if it will comply.

With RCD protection then even if it does read less than 160A for the PSCC then likely it will still trip before damage happens but next EICR it will fail.

 

[Risteard]

B = 5x, C = 10x and D = 20x

Not true. It's 3-5 times for B, 5-10 times for C, and 10-20 times for D.

 

[bernardgreen]

As pointed out the CPC ( earthing provision ) may have to be verified as satisfactory if you change the type or current rating of the MCB / RCBO.

Is this a recent addition to your workshop ? Did you notify the DNO about this 3 kilowatt motor being used on a domestic supply ?

If the in rush current causes dips in the supply voltage then neighbours may start asking the DNO why their lights are blinking. They may link the dips in their lights to the sound of your planer starting. That will tell the DNO where the offending motor is located.

 

[JohnW2]

What we have to remember is 230v / 160A = 1.44Ω ....

As of 3 days ago, you have no choice but to multiply that by 0.95, so it's now 1.37Ω. Looked at another way, it is now 218.5V/160A - not quite as low as the (electrically logical) 216.2V/160A (=1.35Ω), but IMO a definite step in the right direction!

Kind Regards, John

 

[JohnW2]

With RCD protection then even if it does read less than 160A for the PSCC then likely it will still trip before damage happens ....

What will "likely still trip before damage happens" the RCD or the MCB)? The presence of an RCD will obviously not have any influence on the operation of an OPD in response to over-current.

Kind Regards, John

 

[ericmark]

I accept that now we take voltage as 218.5 for calculations. The question of safety is two fold one will line to earth fault cause the supply to auto disconnect so with RCD it needs around a 200Ω impedance or less and with a line to neutral fault will it auto disconnect and for that we with a B16 require a 1.35Ω impedance or less or as you say 160 / 0.95 = 168.4 amp prospective short circuit current or more.

With meters with the BS1363 to C13 IEC connector it seems there is no set standard. Some auto swap between earth - line and neutral - line when switched from loop impedance to PSCC and others don't with the latter the user has to make up his own lead set to test or use the test probes. I must admit after using top of range Robin which did swap line - earth to line - neutral when testing PSCC when I moved to a cheaper meter I never even thought about it until I came to work on TT and noted a very low PSCC.

I agree the MCB letters are a range C = times 5 to times 10 but we for the PSCC are only interested in the upper limit. So yes a C16 = 80 ~ 160 amp where a B32 = 96 ~ 160 amp so in theroy a C16 could trip before the B32 but in practice I have found where run amps are less than 16 swapping a B32 for a C16 has not caused a problem.

As to dips in the supply even a fridge/freezer can cause lights to dip when they start using a plug in energy meter I found my old units used 10 amp on start if this is a problem then one has to consider using three phase motors with an inverter drive. I do wonder if a resistor start can be used with single phase motors? I have never tried it. I suppose in theroy it should work but unless the lights are dipping on start I would not worry about it.

The point is however it's not as simple as swapping a B16 for a C16 first one needs to test the supply to see if a C16 will be within the laid down limits. Since loop impedance testers are expensive this is not really a DIY job.

 

[Adam_151]

With meters with the BS1363 to C13 IEC connector it seems there is no set standard. Some auto swap between earth - line and neutral - line when switched from loop impedance to PSCC and others don't with the latter the user has to make up his own lead set to test or use the test probes. I must admit after using top of range Robin which did swap line - earth to line - neutral when testing PSCC when I moved to a cheaper meter I never even thought about it until I came to work on TT and noted a very low PSCC.

been caught out with one of the newer metrels with that, was giving me the L-N loop instead of the L-E loop and didnt have a clear indication that it was doing so!

My own megger LTW is much better in that regard, I just have two standard 4mm lead connections on the back, and a 13A pugtop lead to 3 coloured shrouded 4mm plugs coloured red, green and blue ( http://www.tester.co.uk/media/catal...m/e/megger-sia10-socket-interface-adaptor.jpg) and also a set of probed leads. It will simply do the loop test between whatever two connections are presented, whether that be L-N, L-E, Ph-Ph. The PFC button will then do the pfc calculation for the displayed loop and the stored measured voltage for the previous test and will toggle back and forward between displaying the loop reading and displaying the calculated pfc

 

[JohnW2]

I accept that now we take voltage as 218.5 for calculations. The question of safety is two fold one will line to earth fault cause the supply to auto disconnect so with RCD it needs around a 200Ω impedance or less and with a line to neutral fault will it auto disconnect and for that we with a B16 require a 1.35Ω impedance or less or as you say 160 / 0.95 = 168.4 amp prospective short circuit current or more.

[presumably you mean C16 (or B32), rather than B16, and also 1.37Ω, rather than 1.35Ω]
Yes, but you surely can't get away from the fact that 433.1.1(iii) requires satisfactory disconnection (to protect cables) in response to (L-N) overloads, so the PSCC in that case has to be at least 160A at 218.5V (i.e L-N loop impedance ≤1.37Ω), whether you have an RCD or not (since RCD won't help in the case of an L-N overload).

As I see it, only in the very marginal situation of the PEFR being >1.37Ω (hence too high for adequate fault protection by MCB), but the PSCC being (as required) ≤1.37Ω would the presence of an RCD make any difference (from this point of view) - and (in a TN installation) even that only if you regard it as acceptable to 'rely on an RCD' for fault protection. .... or am I missing something?

Kind Regards, John

 

[ericmark]

Thank you John well spotted. Yes meant C16.

Yes line - neutral 1.37Ω or 168.4 amp but this is line - neutral with a line - earth fault then with a RCD the 200Ω comes in. I have been caught out with meters and after using a Robin which swapped between line - neutral and line - earth as you swapped between loop impedance and PSCC I expected the next meter to do the same. However when I got to a TT installation it read PSCC as 5A which was clearly wrong. I think Adam's meter is the best option having a plug with three sockets for the two meter leads so the user selects is a far better idea.

However the main problem is how does a DIY person without a loop impedance meter test the supply. When I wanted to hire when mine went in for calibration I was quoted £75 minimum charge which was for a weeks hire. The problem is the traceable records and when you hire the meter you also need all the calibration certificates which bumps up the price.

So in real terms likely easiest and cheapest method is to employ an electrician with the instrumentation to do the work for you.

 

[JohnW2]

Yes line - neutral 1.37Ω or 168.4 amp but this is line - neutral with a line - earth fault then with a RCD the 200Ω comes in.

Yes, I realise that - but my point is that if the L-N loop impedance is >1.37Ω, the circuit remains non-compliant [in relation to overload (L-N)] even if it is compliant in relation to L-E faults by virtue of the presence of an RCD (if one is happy to 'rely on an RCD' for this).

I'm not quite sure why you talk about 200Ω - from the point-of-view of RCD operation, it can be as high as 7,667Ω (at 230V, 7,283Ω at 218.5V)

I have been caught out with meters and after using a Robin which swapped between line - neutral and line - earth as you swapped between loop impedance and PSCC I expected the next meter to do the same. However when I got to a TT installation it read PSCC as 5A which was clearly wrong. I think Adam's meter is the best option having a plug with three sockets for the two meter leads so the user selects is a far better idea.

Provided one looks at the screen, it's pretty foolproof with Fluke MFT's. Pressing a button toggles it backwards and forwards between L-N and L-PE, and either "L-N" or "L-PE" appears on the screen. As an added confirmation, for L-N tests the calculated current is displayed as "PSC" for L-N tests and as "PFC" for L-PE tests.

However the main problem is how does a DIY person without a loop impedance meter test the supply. When I wanted to hire when mine went in for calibration I was quoted £75 minimum charge which was for a weeks hire. ... So in real terms likely easiest and cheapest method is to employ an electrician with the instrumentation to do the work for you.

Indeed - virtually no 'occasional DIYer' is going to either buy or hire the proper testing kit, particularly for a small job. As a consequence, a few may employ an electrician, but most probably won't. Although the purists will obviously be dissatisfied, at least in the case of a sockets circuit, as you often say, a plug-in tester will give a "0-1.7Ω" reading - far 'better than nothing' and, in reality 1.7Ω will undoubtedly result in OPD operation before any cable damage occurs.

Kind Regards, John

 

[jmac80]

Thanks for all the replies guys.
So i ordered a 20a hager rcbo type c after the first couple of replies but now wondering should i have lol..
Little in depth for me the replies above, Once the new rcbo get shere i'll contact a sparky and ask him to test if it will be ok, what do i need to ask him to do and why in layman's terms?
Cheers