Hut Consumer Unit & MCB Question

The consumer unit in the hut has a 6a and 32a MCBs

In the hut I am running a hobby mill and lathe. The mill has a powered feed for the table and if I quickly switch from forward to backward the MCB trips. However its not the MCB in the hut but the one that the armored cable is attached to in the house which is also 32A.

I am wondering if I should change the MCB in the hut to say a 20a/25a MCB?

Do you know what type of MCBs are in each consumer unit? A type B MCB should trip before a type C, which in turn should trip before a type D of the same rating. If the consumer unit in the house is a type A, and that in the workshop a type B or C, simply swapping them may stop the trips.

That still doesn't solve the cause of the trips, which appears to be a momentary current of at least 90A. I wonder if you could prevent this by pausing before reversing, perhaps using a reversing starter with inbuilt delay?

Brian

Yes waiting for the feed to stop before changing direction is what I do but sometimes forget.

Just don’t want to have to walk back to the house to reset the mcb

They are both type b, so the other alternative was to replace the house one with a type c?

so which would be best, rapace house with type c or replace hut with 20/25a mcb?

Edited By petro1head on 27/09/2018 09:12:34

You would need more details of the circuit before replacing a type B with type C. Possibly the house end mcb is at the low end of the specification tolerances. If they are both the same make you could swap them.

Petro1head,

The cheapest option is to employ a little discipline and pause between switching to reverse the feed; either that or get used to the trek back to the house!

Why not make a cover for the switch instead which you have to lift or move aside to delay the action enough to avoid the problem.

Regards

Brian

Honest guv, I do, but on the odd occasion my exuberance gets the better of me .....................

Removed contents overtaken!

Edited By V8Eng on 27/09/2018 10:19:20

Edited By V8Eng on 27/09/2018 10:47:13

Picking up on Brian's point, I think more information would help to pin down the cause. Is popping the MCB simply caused by a current spike due to abruptly reversing a motor or is something else wrong as well? We don't know.

No clue in the original post about the size of the mill - 500W or 5000W? How much power does the mill's main motor consume, and how powerful is the traversing motor? Is anything else connected to the same MCB?

One way of reducing the 'bang' on throwing a reversing switch is to use a three-way switch with a distinct detent in the central OFF position. Not only does the switch add more physical delay allowing motors to slow down, it reminds the operator not to push his luck when reversing! I've also seen circuits with a relay that broke the power entirely until a reset was pressed : you can do a lot of damage by reversing a machine without allowing it to stop first.

Another solution if you have a SWMBO handy. Tell her you need help breaking a bad habit. Every time you pop the MCB she gives you six of the best with a garden cane. It only works if you don't enjoy that kind of thing...

Dave

Following the motor instructions is, by far, the better (and most obvious) solution. Or common sense, if there are no instructional warnings.

You are at risk of damaging your motor by such tricks, so there is also a financial incentive to follow the sensible route and make sure the drive has stopped before reversing it. The power feed motor is not that powerful and taking most of that 32A will certainly not do it any good.

There is more to this than meets the eye and changing breakers willy nilly is not the solution. Reducing the size of the local MCB will not allow discrimination, that is a fault current that trips a say 20A MCB will likely enough also trip the 32A one in the house. Changing the from a type B to a type C is a possibility but there are additional factors that require determination before that can be done, loop impedance is critical, the size of the supply cable has a bearing as does it's length. Without that information it's impossible to make any sensible suggestion and if the OP can't work that out then an electrician is required.

Regards

Martin

Swap the house one for a D curve breaker and the problem should disappear. No building regs approval needed for this.

The rating and type of the MCB's will, if the system has been properly designed , be correct to protect the cable (at the head end) and separately the installation at the hut end, with discrimination such that a 'local' fault stays just that.. To have MCB's identically sized is not ideal and may be totally incorrect, indicating that the installation was possibly just guesswork.

If you are only using one bit of machinery at a time the 32A MCB in the hut could be sensibly replaced by one rated at 16A but that won't necessarily stop the tripping problem.

A physical check of all connections (phase neutral and earth) from end to end plus checking earth integrity on the equipment is a good idea.

MCB's can be supplied or even go trip happy so a direct 1:1 replacement of all the existing protective devices might possibly 'fix' the problem.

Simply swapping the an MCB from a type B to a type C or even a type D without being fully aware of the implications of such a change is not a sensible idea. It might not provide enough discrimination which is what you are after and it could degrade the effective level of protection on the cable from the head end to the hut end.

But as ever, without having sight of the installation, the earthing arrangements and measurements of the fault level most of this advice is just 'guessing'

I'm sorry gentlemen, but I have already solved the 'problem' without any of these erudite concerns and listed it earlier. I wasn't alone in giving the same advice either.

All the OP has to do is take a pause and let the feed motor run down to stop,

All these high powered observations are worth nothing against just a modicum of patience on his behalf which costs him nothing at all. Why oh why must such an issue escalate into such high powered and rather pointless discussion?

Taking cover now

Brian

Interesting.

Out of interest the small hut consumer unit came fitted with the 32a mcb and it never got thought about, but I would agree it’s possible over rated. Considering the plugs are 13a, maybe, from a safety point of view, the mcb should in fact be 16a

however Brian is indeed correct and best practice is patience and waiting for the motor to stop before reversing.

In the UK we use ring mains and a lot of double sockets so the 32A MCB is normally what is required (see the building regs). No harm fitting a smaller MCB but surely that would be the opposite of an improvement.

Murray

The MCB's , like fuses should always be rated to protect the cable. An appliance you plug into a '13A" socket will often as not have a flexible cable rated at lower than that, hence lower fusing in plugs such as 3A. Meanwhile in the rest of the world, with unfused plugs and radial circuits such devices are protected only by a 16A MCB and an RCD.

If you have a ring main run in 2.5mm^2 Twin and earth or singles in conduit then 32A might be appropriate. For a radial circuit (i.e. one cable from the consumer unit to each socket in turn without another cable returning to the consumer unit) again using '13A' sockets or BS4343 'industrial' sockets 16A would often be more appropriate. A 16A MCB might indeed operate before a 32A MCB and a B rated breaker before a C rated breaker before a D rated breaker, but they are not strictly interchangeable and the type and level of fault could mean they all operate together, regardless of rating and trip curve within a few hundred milliseconds and thus provide no effective discrimination

Downrating an MCB from 32A to 16A does not enhance 'personal' protection, the Residual Current Device and the earthing does that. Segregating the lighting and power to equipment is a good thing but a fault on the equipment could still plunge you into darkness, with resultant injury from trip hazards. Hence why RCBO's (a combined MCB and RCD) on each circuit are preferable, further enhanced with the lighting split across two circuits. Of course if you have windows on your workshop and only work in daylight this is not really an issue

Yes you could let the feed stop before reversal but equipment shouldn't routinely trip protective devices regardless of what the user does so this discussion is IMHO far from being 'pointless' and a wrongly sized breaker can be a fire hazard, an incorrectly earthed installation in an outbuilding can be a shock hazard. Because it hasn't killed you in 30 years does not mean it won't kill you tomorrow

I'm afraid you've done nothing of the sort; you've just ignored the problem.

Due to a damaged extension cable I recently had a phase to neutral short in the cable - a bright flash and a big bang. But it didn't trip the 16A breaker in the distribution panel. I regularly switch the motor on my repetition lathe from full speed forward to full speed reverse with no pause. That doesn't trip the breaker either. So why should a hobby power feed trip a bigger breaker? Something is wrong with the wiring installation or the power feed. If it isn't found and fixed it may well cause more serious problems at a later date.

Andrew

The primary purpose of an MCB (or fuse) is to protect the wiring so it should be rated to the wiring. A 32A MCB is only appropriate to a ring main wired with 2.5mm Twin and earth cable or a spur wired with heavier cable. So if the 32A MCB in the hut only has one 2.5mm wire connected to its output it's oversized. If this is the case change it to a 16A MCB MAXIMUM. The size of the one in the house depends on the cable feeding the hut. It should be the same or larger rating than the largest in the hut without exceeding the cable rating. I'd hope the hut is fed with 3 core 2.5mm Steel Wire Armoured (SWA) cable. if so a 32A breaker is OK. If it's just a length of 2.5 twin and earth you need to change the breaker to a 16A and think about a new feeder.

You should really use a "C" rated MCB on a motor circuit but has been mentioned the earth impedance needs to be low enough to allow it to trip quickly in the case of a short circuit. This really needs a measurement.

The other easy and safe fix is to fit a 16A "B" MCB in the house and a 10A "B" or 6A "C" MCB in the hut.

Robert.