• We invite you to join UKWorkshop.
    Members can turn off viewing Ads!

MCB Tripping

UKworkshop.co.uk

Help Support UKworkshop.co.uk:

50020

Established Member
Joined
8 Aug 2020
Messages
29
Reaction score
5
Location
North Yorkshire
My MCB for my workshop/garage is tripping, at what I would expect to be less that the rating of the 16A breaker.

In the house CU, there is a 16A MCB for the garage. In the garage, I have a CU, with a 16A MCB for my workshop.

When for example I have the Vacuum on for dust (the 1100W High Filtration Dust Collector) when I start my table saw, the DeWalt 7485 which is 1850W the MCB in the house trips. No matter which way round I start them up, the MCB in the house trips. There is no other load on the MCB, other that the two items.

Being nearer, I would have at least expected the MCB in the garage to trip, but this isn't the case.

2950W, at ~240V I make just over 12A. Obviously I know the start up current might be higher - but should I be expecting this to trip?
 

sploo

Somewhat extinguished member
Joined
8 Nov 2014
Messages
3,200
Reaction score
541
Location
West Yorkshire
Not that unexpected in my experience - though that's personal rather than professional (i.e. I'm not a certified sparky).

I had a very similar issue, and replacing the MCB in the house with a Type C of the same rating solved the issue.
 

guineafowl21

Established Member
Joined
28 Oct 2015
Messages
510
Reaction score
134
Location
Inverness
My MCB for my workshop/garage is tripping, at what I would expect to be less that the rating of the 16A breaker.

In the house CU, there is a 16A MCB for the garage. In the garage, I have a CU, with a 16A MCB for my workshop.

When for example I have the Vacuum on for dust (the 1100W High Filtration Dust Collector) when I start my table saw, the DeWalt 7485 which is 1850W the MCB in the house trips. No matter which way round I start them up, the MCB in the house trips. There is no other load on the MCB, other that the two items.

Being nearer, I would have at least expected the MCB in the garage to trip, but this isn't the case.

2950W, at ~240V I make just over 12A. Obviously I know the start up current might be higher - but should I be expecting this to trip?
Having identically rated devices far apart like that is a poor choice, as you’ve found - the protection should gradually step down in value towards the point of use, so the most likely thing to blow is the thing nearest.

There are two tripping methods - overload (long term) or thermal trip, and magnetic (inrush/startup) trip. You’re getting the latter, by the sound of it. The actual current value is not as important as the rate of rise in these cases.

The house MCB is there to protect the cabling to the garage, up to the garage CU. You could either uprate the current rating, say to 20A, or the trip characteristic, say from B to C. You’ll need an electrician to assess what cabling is there, and which option to choose based on measured values such as loop impedance.
 
Last edited:

Chris Hawkins

Established Member
Joined
23 Feb 2019
Messages
27
Reaction score
20
Location
Chippenham
I am not qualified to offer advice. Nevertheless, I found that switching from a Type B to Type C MCB solved a very similar problem.
 

50020

Established Member
Joined
8 Aug 2020
Messages
29
Reaction score
5
Location
North Yorkshire
The house MCB is there to protect the cabling to the garage, up to the garage CU. You could either uprate the current rating, say to 20A, or the trip characteristic, say from B to C. You’ll need an electrician to assess what cabling is there, and which option to choose based on measured values such as loop impedance.
The cable is easily identifiable in the Garage CU, I'll check. If it's 1.5mm-sq then 16A really would be the limit, without expensive re-cabling.

I was kind of hoping the 16A MCB in the house might just be a bit over-sensitive, as I've known that in the past, but not where a further CU is fed.

The manufacturer of the MCBs and CU in the house is Legrand, and the cost of C Curve 16A MCBs is extortionate! I've also not had good experience when I have renewed/replaced MCBs from one manufacturer to a different manufacturer of the CU, as there was slight differences which prevented the cover being fitted :confused:
 

guineafowl21

Established Member
Joined
28 Oct 2015
Messages
510
Reaction score
134
Location
Inverness
The cable is easily identifiable in the Garage CU, I'll check. If it's 1.5mm-sq then 16A really would be the limit, without expensive re-cabling.

I was kind of hoping the 16A MCB in the house might just be a bit over-sensitive, as I've known that in the past, but not where a further CU is fed.

The manufacturer of the MCBs and CU in the house is Legrand, and the cost of C Curve 16A MCBs is extortionate! I've also not had good experience when I have renewed/replaced MCBs from one manufacturer to a different manufacturer of the CU, as there was slight differences which prevented the cover being fitted :confused:
1.5mm2 is generally for lighting circuits. I would go for the re-cabling if that’s the case - hopeless for woodworking machines.

It could be a more sensitive MCB, but don’t just swap in a type C, especially as your cabling may well be on the limit. If the resistance (*impedance) of the cabling is too high, the type C may not trip in the event of a fault.

You are right to stick to the same brand of devices.
 

Sideways

Established Member
Joined
26 Dec 2017
Messages
1,291
Reaction score
207
Location
United Kingdom
I haven't looked but I imagine the dewalt saw is a brushed motor, not an ac induction motor. Induction motors are famous for having inrush currents upto several times the running current but only for a moment. Brushed motors (I think, but have never had reason to measure them myself) have less inrush.
Either way, try plugging your saw in through a couple of long extension cables. Just a little extra resistance can reduce the surge current and may make a difference.
If this works, try replacing the breaker that is tripping like for like. Everything is made to a tolerance. You may just have a "sensitive" one.

If you swap your type B for a Type C. You have meddled with the design of fixed wiring and you are not qualified to do so (or you would know the answer to your own question). That change is a job for a sparky and technically subject to building regs although the harm from any short term overloads you may cause could take a long time to manifest.

Haha, Guineafowl had already said all of that :)
 

Spectric

Established Member
Joined
19 Feb 2015
Messages
1,870
Reaction score
789
Location
North Cumbria
In the house CU, there is a 16A MCB for the garage. In the garage, I have a CU, with a 16A MCB for my workshop.
You have no discrimination.

There is also no mention of how long this cable run is and the type of cable. Also what is after the 16amp Mcb in the garage? Sounds like the design has some issues.

I had a very similar issue, and replacing the MCB in the house with a Type C of the same rating solved the issue.
Bearing in mind that a type C should only supply a single load, one with a high startup current and not another dizzy board. One of the reasons is that they require between five and ten times rated current to trip compared to a type B at five times, so cable length and prospective fault currents need to be well within spec.
 

sploo

Somewhat extinguished member
Joined
8 Nov 2014
Messages
3,200
Reaction score
541
Location
West Yorkshire
Bearing in mind that a type C should only supply a single load, one with a high startup current and not another dizzy board. One of the reasons is that they require between five and ten times rated current to trip compared to a type B at five times, so cable length and prospective fault currents need to be well within spec.
Fair point - the problem is that in my garage I initially installed a Type C MCB in the CU for the ring main (for those high startup current tools) but it was ultimately always the Type B MCB in the house (that supplies the garage CU) that would trip. In that scenario, what is the proper solution?
 

Spectric

Established Member
Joined
19 Feb 2015
Messages
1,870
Reaction score
789
Location
North Cumbria
Your problem is what we call discrimination, it is where you design a system so the protective device nearest to a fault will operate before any upstream protective devices so minimising disruption to other loads. With lets say a 32 amp type B in the house board and a 32 amp type C in the garage then because the type C will require a fault current of about 320 Amps the type B will operate first because it only requires 160 amps so the type C is doing nothing, but this should be just a two pole isolator, not a protective device because there are downstream protective devices in the board to protect the outgoing circuits. When doing a design you would look at the load in the garage, the type of supply to the property and calculate the required type & size of cable. Knowing the cable size allows you to determine the type of protective device needed at the supply end, then in the garage board one of the protective devices could be a type C for a machine with a high inrush current. All this requires the lengths of the cable runs and method of installation because there are a lot of variables and at the end during commisioning you need to measure certain parameters that will be put onto the installation cert and that requires some fairly expensive equipment, albeit a lot less than what is needed for industrial 400 volt three phase systems.
 

NetBlindPaul

Established Member
Joined
23 Jun 2009
Messages
302
Reaction score
55
Location
South Wales
As Spectric suggests there is a lot more to this than just randomly changing circuit breakers.
Also, if you change from a B to a C curve breaker at 16 A you will not achieve selectivity.
We are not allowed to use the term discrimination any longer it’s considered an indiscretion.
Also in going from a B to a C you will need around half the earth fault loop impedance to ensure that you achieve disconnection in the event of a fault for shock and fire prevention reasons.
A B16 requires a design EFLI of 2.73 Ohms; a working (measured) EFLI of 2.28 Ohms
A C16 requires a design EFLI of 1.37 Ohms; a working (measured) EFLI of 1.14 Ohms
For reasonable selectivity with a B16, you would need to change the upstream device for a C32.
A C32 requires a design EFLI of 0.68 Ohms; a working (measured) EFLI of 0.57 Ohms
This C32/B16 combination would scrape selectivity if you have a 63A incoming fuse, might be a 60, 63, 80 or 100A, without opening up the DNO cutout, which is both dangerous and illegal, or asking the DNO it is not possible to tell.
For the device to be changed the supply, installation, and circuit characteristics must be assessed for safety, for example, earthing & bonding, supply type, installed current carrying capacity of the sub main to the garage, etc., etc.
If, the installation and circuit are suitable for modification, then the EFLI needs to be confirmed by design and verified upon completion.
This isn’t a fuss or not necessary because it’s DIY, this is critical for safety, and if not done corectly you might find your house insurance invalid.
For example, picking the cable size mentioned above at 1.5mm sq., IF, I were employed as the expert witness to investigate the electical aspects of a house fire at your property and I could identify a 1.5mm sq. cable connected to a C32 breaker, then I would be advising the insurance company to refuse payout unless you could provide an EIC giving the name of the persons who designed, constructed and inspected and tested the circuit, guiding you to claim off their insurance.
 

Attachments

mikej460

Established Member
Joined
19 Jan 2019
Messages
560
Reaction score
302
Location
Daventry
If you haven't already then check all terminals for tightness and ensure none are pinching insulation. Also make sure both mcbs negative terminals are correctly seated against the busbar. I had a problem once where this was the cause of erratic tripping under load. Having said all that it does sound like you need to recalculate your loading and design as per previous posts.
 

50020

Established Member
Joined
8 Aug 2020
Messages
29
Reaction score
5
Location
North Yorkshire
1.5mm2 is generally for lighting circuits. I would go for the re-cabling if that’s the case - hopeless for woodworking machines.

It could be a more sensitive MCB, but don’t just swap in a type C, especially as your cabling may well be on the limit. If the resistance (*impedance) of the cabling is too high, the type C may not trip in the event of a fault.

You are right to stick to the same brand of devices.
I did check the cable size, and it's 2.5mm2 which the builder installed to the garage. This fed a garage CU with a 63A breaker (Never understood why a garage CU would have a larger rated MCB than the supply from the house?). Garage Sockets were installed on a 20A MCB, and lighting on a 6AMCB. I added the 16A breaker for the workshop.

I am not keen on idea of swapping the type of MCB (from B to C) as mentioned, this changes the design. Something I am not qualified to do, but more importantly to me, don't have the experience to calculate the effects of such.

I may when I get chance swap the MCB for one of the exact same (in case it's just a sensitive breaker), but not having both running for now has not been a problem really, it's just a nice to have.

Thanks to all contributors so far though.
 

clogs

just can't decide
Joined
24 Jul 2020
Messages
720
Reaction score
331
Location
Vamos, Crete, GREECE.......
also MCB get tired...
had something similar here......I was using a small buzz welder, which runs happily on a 13amp English plug, the fuse in the plug was never blown just the MCB.....didn't want to change the rating so for the hell of it just bought a new MCB.....
and the tripping fault stopped......
can't remember the name of the old one but the new one was Siemens.....
 

guineafowl21

Established Member
Joined
28 Oct 2015
Messages
510
Reaction score
134
Location
Inverness
I did check the cable size, and it's 2.5mm2 which the builder installed to the garage. This fed a garage CU with a 63A breaker (Never understood why a garage CU would have a larger rated MCB than the supply from the house?). Garage Sockets were installed on a 20A MCB, and lighting on a 6AMCB. I added the 16A breaker for the workshop.

I am not keen on idea of swapping the type of MCB (from B to C) as mentioned, this changes the design. Something I am not qualified to do, but more importantly to me, don't have the experience to calculate the effects of such.

I may when I get chance swap the MCB for one of the exact same (in case it's just a sensitive breaker), but not having both running for now has not been a problem really, it's just a nice to have.

Thanks to all contributors so far though.
Let us know how you get on.

Is it definitely a 63A MCB, and not just an isolator switch or RCD rated to handle 63A? Take a pic if unsure.
 

Spectric

Established Member
Joined
19 Feb 2015
Messages
1,870
Reaction score
789
Location
North Cumbria
How long is your cable run as with 2.5 you may find that you are getting a large volt drop under load,
This fed a garage CU with a 63A breaker
The maximum size protective device on 2.5 is 20 amps, so the house 16 amp is ok but the garage 63 is doing nothing, it should only be a two pole isolator for the other outgoing circuits.

I would never run a 2.5 as it is to small, 6mm more common and in some cases with PME supplys a 10mm but then use earth rods.

You should keep builders away from electricity, bricks are more than enough to keep them busy.
 

Spectric

Established Member
Joined
19 Feb 2015
Messages
1,870
Reaction score
789
Location
North Cumbria
Sorry, yes it's an isolator switch, doh!
So the basics are correct, the 16 amp protects down to that isolator, all protective devices downstream of this isolator should be less than 16 amps, ideally 10 amps, so you have lighting. You could fit a 20 amp in the house consumer unit depending upon the impedance of that cable which depends on how long that run is, it may be that the run is such that the impedance is to high for the use of a 20 amp protective device, also depending on type of cable used the CPC may also be to small. So type of cable and length of run would help and even better if you could measure the PSCC & PEFC but you need a decent multi function tester. You actual problem is that cable size is to small to feed a downstream dizzy board, it could have a couple of double 13 amp sockets on it and all be protected by the 16 amp in the house buts that it.
 

50020

Established Member
Joined
8 Aug 2020
Messages
29
Reaction score
5
Location
North Yorkshire
How long is your cable run as with 2.5 you may find that you are getting a large volt drop under load,

The maximum size protective device on 2.5 is 20 amps, so the house 16 amp is ok but the garage 63 is doing nothing, it should only be a two pole isolator for the other outgoing circuits.

I would never run a 2.5 as it is to small, 6mm more common and in some cases with PME supplys a 10mm but then use earth rods.

You should keep builders away from electricity, bricks are more than enough to keep them busy.
The cable run to the garage is in the region of 15-20m. It is a two pole isolator, I was mistaken calling it an MCB.

It's a new build home, so had no control over what the builder installed. I suppose 2.5mm2 for 20A is more than enough for most home applications, the fact that there is a 16A mcb in the house - I am sure that this could be uprated to a 20A one, but that is something that I am not going to do, but would get a qualified electrician to do.
 
Top