Hut Consumer Unit & MCB Question

[petro1headParticipant @petro1head]

Correct well spotted but its still armoured and where it runs would never be attacked by anything

As you said, can of worms. Enough said

[Mark RandParticipant @markrand96270]

Just seen the cable size. 2.5mm^2 SY is only rated at a maximum of 24A and will be less than that for any distance due to voltage drop. So breakers should be sized accordingly. 4mm^2 SWA would be the minimum for a 20 metre outdoor run and 32A breakers.

The 2.5mm^2 SY can be re-used for connecting individual machines to their plugs, that's what it's designed for.

A D curve breaker will not affect the cable rating.

[Former MemberParticipant @formermember19781]

[This posting has been removed]

[petro1headParticipant @petro1head]

I kind of wish I had never asked here but on the other hand I know I would get good info. Maybe more than I wanted but good for anyone else thinking aboiut doing the same

I think we need to get this into perspective.

I run either the lathe or the mill, never both. Not enough hands

The cable is more than capable of doing its job, its not like I am trying to run a constant 3KW!!

[Robert Atkinson 2Participant @robertatkinson2]

My earlier post crossed with Martins.

The SY cable is not suitable for this application especially if buried. A 32A breaker will not protect this. You need to change the breaker in the house to a 16A one as soon s possible. (a 20A might do but but you would need to know the loop impedance. Change the hut one to 10 or 16A. Even SWA comes in different current ratings depending on the plastic used for the insuation. I disagree with the comment that cable size is not affected by MCB type (B,C or D), it does indirectly becaus it affects the loop impedance. The loop impedance must be low enough for the MCB to trip quickly with a short circuit. https://www.electricalsafetyfirst.org.uk/mediafile/100106752/master_EARTHFAULTLOOPtable.pdf
If you have a long cable and a C or D MCB you may have to go down in MCB rating or up in wire size.

Sounds like you need to get a electrician in, trouble is finding one that knos what they are doing but won't do more than is really needed.

Robert.

 

Edited By Robert Atkinson 2 on 27/09/2018 17:29:33

[Robert Atkinson 2Participant @robertatkinson2]

Posted by Bill Chugg on 27/09/2018 16:55:11:

My workshop was 5 metres from the house at my last property.

Electrician used 6mm SWA as supply as he said that was the minimum he could use to comply with the regulations.

Thats just wrong, 6mm SWA is rated for at least 50A. A workshop feed should not need more than 32A. 2.5mm would do for a short run and 4mm would be conservative. Too late now though.

Robert.

[petro1headParticipant @petro1head]

Posted by Robert Atkinson 2 on 27/09/2018 17:50:01:

Posted by Bill Chugg on 27/09/2018 16:55:11:

My workshop was 5 metres from the house at my last property.

Electrician used 6mm SWA as supply as he said that was the minimum he could use to comply with the regulations.

Thats just wrong, 6mm SWA is rated for at least 50A. A workshop feed should not need more than 32A. 2.5mm would do for a short run and 4mm would be conservative. Too late now though.

Robert.

I was also thinking that but glad you said it

[Mike PooleParticipant @mikepoole82104]

I don’t think a workshop supply being over cabled will be a problem, as a boy my workshop was at the top of the garden and powered by a very long extension cable, starting a motor dropped the voltage enough to induce severe flicker in the fluorescent tube light, once running the motor and light were happy. The startup surge should be considered if minimalist cabling is installed. The regs have plenty to say about cable size and installation conditions and breaker sizing to give proper discrimination. A proper design exercise should be done to determine what cable and breakers are required for every installation, guessing will result in an inadequate installation or wasting money on over cabling.

Mike

[Mark RandParticipant @markrand96270]

Posted by Robert Atkinson 2 on 27/09/2018 17:50:01:

Posted by Bill Chugg on 27/09/2018 16:55:11:

My workshop was 5 metres from the house at my last property.

Electrician used 6mm SWA as supply as he said that was the minimum he could use to comply with the regulations.

Thats just wrong, 6mm SWA is rated for at least 50A. A workshop feed should not need more than 32A. 2.5mm would do for a short run and 4mm would be conservative. Too late now though.

Robert.

Sorry, but that isn't the case. It's rated at 50A current for a short length. You'd need 10mm^2 for 50A over 20 metres buried in earth. You have to plan on voltage drop as well.

I did 16mm^2 over 30m to the shed, with a 40A circuit to allow me to go to 63A if needed.

[EmgeeParticipant @emgee]

Posted by Robert Atkinson 2 on 27/09/2018 17:50:01:

Posted by Bill Chugg on 27/09/2018 16:55:11:

My workshop was 5 metres from the house at my last property.

Electrician used 6mm SWA as supply as he said that was the minimum he could use to comply with the regulations.

Thats just wrong, 6mm SWA is rated for at least 50A. A workshop feed should not need more than 32A. 2.5mm would do for a short run and 4mm would be conservative. Too late now though.

Robert.

I'm sure there are many forum users who have machines or plant in home workshops that have a need for more than a 32A supply so I believe the advice to use 6mm SWA was sensible, even for such a short run, the cost difference was negligible.
There will also be many who experience the lights dipping or even going off when starting machines or plant, some of those will have undersize supply cables.

Perhaps the cable was routed through a boiler room with high ambient temperature and both walls where the cable passed totally enclosed the cable in insulation, there may also be any number of live circuits on part of the route.

Without knowing the exact details of the installation it is impossible to say the cable is not a good choice.

Emgee

[john fletcher 1Participant @johnfletcher1]

The all important thing with any circuit is the disconnection time. If the circuit protective conductor (CPC) or in old terms the earth wire, is small then it will take a fault current longer to rise to a sufficiently high level to disconnect the circuit under fault conditions (High opposition to current flow). Whereas a CPC with a large cross sectional area will have less opposition to fault current flow and will therefore disconnect the circuit more quickly, before a dangerous situation arises. All very similar to water pipes and quantity of water flow. An earth loop impedance test should be carried out and with that information a decision will be made as to what type of circuit protection should be used. Then one has to consider volts drop, and that is determined by the size of the main conductors. milli volts per amp, per metre length of the cable, all divided by 1000, this information is obtainable from the wiring regs, which will soon be the 18^th Edition. So it’s not simply 2.5mm or 6 mm so many variables to be considered. Certainly not many home workshops will actually need 6mm cable unless it’s a long way from the main service. John

[Phil WhitleyParticipant @philwhitley94135]

Most of the above is correct, in certain circumstances, but as Woody says, Totally irrelevant! The tripping is caused by slamming a motor into reverse while it is still running forwards, thus creating a massive torque reaction, and a current spike large enough to trip the MCB. Does it trip at any other time? NO, then that is the cause! "electricians" also please remember that the IEE/IET regulations are NOT law, only the requirements of the building regulations as pertaining to part P thereof is law. As far as all these supposed improvements in electrical safety brought in by advent of the MCB/RCD era, I would just say that we live in interesting times, and time will tell if safety has actually been increased, or in fact reduced.

[EmgeeParticipant @emgee]

John

You are only considering live to earth fault current, phase-phase and phase-neutral fault must also be considered when selecting the Circuit Protective Device to prevent temperature rise damaging the cable.

The EFLI has to be calculated in the circuit design stage to ensure correct PD and cable size are used, confirmed after installation by an EFLI test before the circuit is put into use.

As stated earlier without knowing the full details of requirements it is not possible to advise a cable type/size.

Emgee

[petro1headParticipant @petro1head]

Posted by Phil Whitley on 27/09/2018 19:30:40:

Most of the above is correct, in certain circumstances, but as Woody says, Totally irrelevant! The tripping is caused by slamming a motor into reverse while it is still running forwards, thus creating a massive torque reaction, and a current spike large enough to trip the MCB. Does it trip at any other time? NO, then that is the cause! "electricians" also please remember that the IEE/IET regulations are NOT law, only the requirements of the building regulations as pertaining to part P thereof is law. As far as all these supposed improvements in electrical safety brought in by advent of the MCB/RCD era, I would just say that we live in interesting times, and time will tell if safety has actually been increased, or in fact reduced.

Here here

[Roger BParticipant @rogerb61624]

There are two problems here: 1) the installation may not be fully in accordance with BS 7671 and Part P (depending on where in the UK it is). 2) the breaker trips when the motor is reversed.

1) Not enough information.

2) Making an assumption that the motor is connected via a 13A fused plug and the 13A fuse is not blowing then the 32A mcb is tripping in the instantaneous part of the curve. This suggests a short circuit rather than an overload so either the reversing switch is not suitable for purpose or it is faulty, creating a short when directly switched.

[petro1headParticipant @petro1head]

Roger what information do you want?

The reason the MCB trips but the fuse does not blow is because the MCB has a quicker disconnection time than the fuse so before the fuse has a chance to break the current has already been stopped.

[SillyOldDufferModerator @sillyoldduffer]

My Tuppence worth:

1. The mill is a WM18 with an 1100w motor. It's not a monster.
2. The traversing motor is a commercial unit sold by Warco. Few electrical details on their website – it's 110V and comes with a transformer. Judging by the photo it's not a big motor, probably less than, say, 100W.
3. I would expect the mill and traversing motor together to draw less than 6A, which is well within 13A, let alone 32.

Conclusions

1. On the face of it the electrical wiring of petrolheads workshop isn't the problem:
   • The incoming cable may not be ideal but it's probably not the cause.#
   • Nor would the ring and consumer unit inside the shed produce the symptoms.
2. The power consumption of equipment in the shed is moderate, less than 13A street legal, and shouldn't pop a 32A MCB.
3. The fault occurs when the commercial traversing unit is reversed, which suggests a problem with it.

Next steps:

1. Read the manual carefully to see what it says about reversing. (Possibly the unit has been damaged by operator error, possibly it should power down automatically and isn't due to a fault.)
2. Talk to Warco, especially if the item is within warranty.

# Bound to be possibilities I don't understand! For instance, I'm not sure how an MCB works. The obvious way is to detect excessive current by heating a bimetallic strip or with a magnet coil that operates a mechanical switch. As modern electronics offers other possibilities, could it be that the petrolhead has an MCB that detects voltage sag caused by the thinnish connecting cable and triggers due to that? (In other words a false alarm.)

Dave

Edited By SillyOldDuffer on 28/09/2018 10:10:32

[petro1headParticipant @petro1head]

SillyOldDuffer.

Very sensible reply.

I no longer have the instruction but would hazard a guess it says "wait for the motor to stop before reversing"

Its done this from day 1

Just spoke to Warco and they say, as a few here have said, be patient and wait for the motor to stop before reversing

[Roger BParticipant @rogerb61624]

To determine if the Installation meets the requirements of BS7671 would require PSCC (prospective short circuit current) and EFLI (earth fault loop impedance) measurements. The type of cable used is an open discussion point, SY is not recognised in the Wiring Regulations. This however is not your present problem.

MCBs have two operating elements, a magnetic trip to deal with short circuit conditions and a thermal trip to deal with overloads. Under short circuit conditions they do not discriminate well with fuses or MCBs of different ratings.

The website information for the traverse drive states that it has overload protection. This should protect against you switching direction without stopping. Either this is faulty or there is another fault with the system (such as the switch itself) putting a short circuit on the supply.

[petro1headParticipant @petro1head]

Yes it has overload protection but this is in case the motor stalls, ie the work hits something and stops the bed from moving.

One thing hat has come to light is the MCB at the house end needs to be 20a and not 32a

Anyway I feel this had been done to death and as I just need to be patient

Edited By petro1head on 28/09/2018 10:54:33

[Robert Atkinson 2Participant @robertatkinson2]

Posted by Mark Rand on 27/09/2018 18:50:08:

Posted by Robert Atkinson 2 on 27/09/2018 17:50:01:

Posted by Bill Chugg on 27/09/2018 16:55:11:

My workshop was 5 metres from the house at my last property.

Electrician used 6mm SWA as supply as he said that was the minimum he could use to comply with the regulations.

Thats just wrong, 6mm SWA is rated for at least 50A. A workshop feed should not need more than 32A. 2.5mm would do for a short run and 4mm would be conservative. Too late now though.

Robert.

Sorry, but that isn't the case. It's rated at 50A current for a short length. You'd need 10mm^2 for 50A over 20 metres buried in earth. You have to plan on voltage drop as well.

I did 16mm^2 over 30m to the shed, with a 40A circuit to allow me to go to 63A if needed.

The current rating of a cable has NOTHING to do with it's length. The only consideration is temperature rise and subsequent insulation damage. You can run the same conductor size at higher current if the insulation is rated to a higher temperature. Voltage drop is another matter and a larger size may be required to keep this (and the related loop impedance) within limits. Note that having a low loop impedance can cause as much trouble as high. The Prospective Fault Current (PSC) goes up as impedance goes down and can force the use of proctive deivice with high interruting current ratings pushing you to more expensive industral switchgear. The feeder in question was only 5m long and while the OP did not say, I'd be suprised if the demand was for more than 32A. Additionally few homes in the UK have more than a 100A incoming feed (many are 60A) so even with diversity a workshop feed taking more than 32A leaves little for the rest of the house.

Robert.

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