Tripping Breaker welding

Gallery, Projects and General > How do I??

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Darren:
BTW, I do still get a bit of volt drop at times, it's not constant but the lathe lets me know when. That's in 2HP mode and cold, it's fine when it's warmed up a bit .... usually.

Fred Bloggs:
Darren

I'll work some numbers with the info you've posted and get back asap work / SWMBO permitting :thumbup:

Fred

Darren:
Today I plugged one of those electricity monitors into the socket outlet for the welder. I don't know how accurate these things are but it will give some idea of what is going on.

This measures what is happening on the 240V side of things.

The readings were on full welding power.

238V

3.4KW

19A

Though this is within the scope of a 32A MCB I'm guessing it still is not telling us what the surge current is as the trip does it's job a bit quick for the plug in tester to take a reading. But as the trip behaved itself mostly today those readings are under continuous welding at full power.

If I connect the welder to the house socket with only an RCD breaker on a long 2.5mm lead about 35m all works fine. i.e. no tripping.

I really do think it's just the wrong spec MCB for an induction device such as the welder. This problem is quite common.

Fred Bloggs:
Darren

Apologises for taking a while to get back to you, Family and work and a bad back have priority. Please see the calculations below, Note that due to me not knowing the site I have made some broad assumptions with regard to the electrical characteristics at your property!!

As you have stated that you are feed from an overhead line system (TT system for the electrically minded among you)therefore must assume that the Prospective Fault current at you meter tails is going to be around 600-700Amps – best case and the Earth loop impedance will be in the region of 21 ohms
I do not know what protective devices are fitted upstream of the SWA cable (Steel wire armoured) feeding the garage (Man kennel) so I will have to ignore them for the purpose of the following calculations – finger in the wind guesses??? – ( lots of shouts of sack him at this point from all the PhD’s in electrical engineering on this site)

Calculations

1/ - (Quick check total volts drop is less than 11.5volts (5% of supply))

Assume that 20amps load on system.

1/a/ Ring main is 18mV/Amp/Meter for 2.5mm² T&E (Twin and Earth Cable)

Therefore Vdrop in ring = (18mV x 20A x 30M)/4
     = 2.7Volts

1/b/ SWA Cable is 12mV/Amp/Meter for 4mm² SWA

Therefore Vdrop in SWA = (12mV x 20A x 30M)/2
     = 5.4Volts

Therefore total volts drop at around 20A load is 8.1 Volts so even if load was actually at 25A due to lights etc in garage volts drop would be 9.75Volts and at 30A load it would be 10.8Volts . which is still (just) under the 11.5V limit.

2/ - Calculation of fault currents and MCB operating times

2/a/ Assume Impedance of the supply is 0.4ohms.

Impedance of the 4mm² SWA cable is 4.61ohms/1000M

Therefore Impedance for 2 runs of 4mm² SWA in parallel,

Length = 45M       Zswa= 0.207ohms

Impedance of the 2.5mm² T&E cable is 19.51ohms/1000M

Therefore Impedance for 2.5mm² T&E cable in a ring main,

Length = 30M      Zring = (19.51mohms/M x 30M)/4
        =0.146ohms

Therefore total Impedance    Zt = 0.4 + 0.207 + 0.146
     = 0.753ohms

But you must apply temperature correction factor of 1.2

Therefore       Zt = 0.753 x 1.2
        = 0.9036ohms

2/b/The Fault current at the furthest point of the ring main should be

         Ifault = 238V (measured)/0.9036ohms
           = 265A

From the Wiring Regs (BS7671) the current required to cause a BSEN 60898 Type B 32A MCB to operate in less than 0.4Secs (Requirement of Reg 411.3.2 – portable equipment) is 160A Therefore as the calculated (Caveats noted) fault current is greater than this by 10% (my safety factor), the system as it is complies with BS 7671.

BUT if you replace the existing Type B 32A MCB with a Type C 32A MCB you will not comply with the wiring regs as the Type C 32A MCB requires a fault current of greater than 320A to operate in less than 0.4Secs.!

SOLUTION

Therefore I suggest and it is only a suggestion that you wire a short piece (3meters) of minimum size 6mm² T&E to a 32A commando socket with a suitable isolator and fit a Type C 25A MCB in a separate way in the garage Consumer unit!

This would give a temperature corrected impedance of 0.756ohms

Therefore fault current would be - If = 238V (measured)/0.756ohms
           = 314A

This fault current is greater than the current required to operate the Type C 25A MCB, which is 250A ,again this would then comply with BS 7671.

Caveat

Please be aware that the above calculations are based on the limited information I have been given, a site survey might conclude that the exist installed system does not even meet current regulations!!!! That is not a critisim of your wiring skills Darren, just that you have quite long cable runs on a overhead TT system in a rural setting , to quote you “I do still get a bit of volt drop at times, it's not constant but the lathe lets me know when. That's in 2HP mode and cold, it's fine when it's warmed up a bit .... usually.”

For example when the lathe is starting as above there is probably 100A inrush for a few cycles of the mains , given a peak volts drop of approx 30V which is going make the lights dip a bit!!

Anyway I hope the above is of some help – If any of the boffins on this forum wish to comment / tear my thoughts and guess work to pieces feel free :smart:

PS I spent ages typing this lot above up in a word document, nice spacings etc and its all c"&*ed up when I copied it onto the forum, arrh well I've tried to correct it, I hope you can understand it.

Best Regards

Fred

Darren:
Hi Fred,

Thanks for getting back on this. I had a quick scan and basically see where you are coming from. (I think :lol:)

What you are trying to avoid is too much volt drop which would at the same time cause the mains cables to get a bit too hot during a fault overload. If I'm thinking rightly if the cables offer too much resistance this can have serious and adverse effect on the trip devices no mentioning the fact that the device will also be slow to trip, if at all.
Not good if you are personally feeding the electrons to ground :zap:

Yes my lights do dip when a machine is started, as do the lights back at the house. But then this used to happen when the lathe was in the house (basement) without the long run of SWA. Even my little Chinese mill used to cause this effect.

Feeding the SWA is a 32A RCD from the house fuse board. via a very short length of 6mm T&E. Terminated in a metal clad box. The SWA has proper glands at both ends.

I'll have another slower read of your post later and try to digest some greater detail.

BTW, my house is fed by 4mm SWA about 80m where it links with next door and then disappears underground. I have no idea how far it goes then ... but it's a long way.
So by what I can see both our houses are fed by one single 4mm SWA and you could consider our house to be eight bedroom and next door is double the size. In fact part of next door is a totally separate dwelling for their parents.
All from one 4mm SWA.

I considered my 2x4mm SWA to be a little overkill, now I'm wondering if my incoming mains is up to regs !!

Much to ponder ......

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