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Consumer units -how do they work?

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Robin Graham06/01/2017 02:51:32
229 forum posts
23 photos

My excuse for posting this on an an ME forum is pretty flimsy, but it's workshop related so I suppose it may be OK.

I've been given an extractor with a 1kW brushed motor, but it trips the workshop side of of my CU when I start it up. Not just the breaker on the power circuit, it's the rightmost switch which turns everything off.

I've tried googling on CU's but the results are along the lines of 'it's a modern type of fuse box, don't worry your pretty little head about it, call in an electrician',

My best guess from what I've read is that the breakers are somehow like old-fashioned fuses, ie current limiting devices, but the switch on the right detects earth leakage. Is that right? If so it would help in diagnosing the problem with the extractor.

REgards, Rob

XD 35106/01/2017 05:45:20
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467 forum posts
22 photos

A picture of the consumer unit would help but if it is the earth leakage ( rcd ) that is tripping there may be a problem with the motor .

Is the extractor new or used ?

Nick_G06/01/2017 07:49:48
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1337 forum posts
527 photos

.

"How do they work" ?? Put in it's most simple form :-

The circuit breakers give overload protection to the individual circuits between live and neutral and prevent damage to the individual circuits should load be exceeded.

The RCD will give protection against current flowing to earth. These are most often set to trip at 10, 30, 50 and 100mA. with 30 being the most popular in a domestic installation.

Circuit breakers will also react to a 'serious' Live - earth fault. But if an RCD is also installed that will obviously additionally trip.

RCD's can sometimes be 'twitchey' devices with seemingly a mind of their own at times. This however, unless the unit is faulty will usually have an explaination .................... Somewhere.!

RCD's can often/will/should trip if there is an neural - earth fault in the property or appliance plugged into it that has a fault. This to the customer may or may not reveal itself until load is drawn through the circuit. This can sometimes be misleading as the item that you think is causing it to trip may not actually be the guilty item.

RCD's are a safety item. Their main purpose is to protect humans from electrocution which is why their trip value is set in the low mA range. They will do this with a live - earth contact but not a live to neutral contact. But as most humans are usually whacked via a path to earth and not neutral it's not such an issue.

Nick

Toby06/01/2017 07:58:15
101 forum posts
17 photos

you are correct Rob, the individual (narrower) switches are over current breakers (called MCBs or miniature circuit breakers) and the one at the end (normally double width) is an RCD (residual current device or earth leakage breaker). The RCD being in series with all the MCBs.

The RCD is almost certainly 30mA rated so is specced to trip if there is 30mA flowing to earth for more than 200ms or 40ms if more than 150mA. Often the actual leakage required to trip it is as low as 20 to 25mA.

This leakage is measured as a difference between the line and neutral currents, the assumption being that what is "missing" must have flowed to earth. So the trip can be caused by either a line to earth fault or a neutral to earth fault (neutral is rarely at exactly earth potential).

There are two main reasons why this can happen.

1) a capacitive link between live (line or neutral) and earth. This can be just capacitance in motor windings or it can be caused by a fault.

2) a low resistance to earth from the line or neutral, normally caused by a fault or breakdown in insulation. Quite often caused by damp or water ingress. Or, in the case of a motor a short (or partial short) of a winding to the motor core.

Unfortunately these things are hard to find without specialist test kit. You need to be able to measure insulation resistance at at least 250V and also measure mA leakage currents with peak hold to detect startup pulses.

One thing to watch is that it might not be just the fan that is causing the problem. There could be (in fact probably is...) a general earth leakage from other devices on the affected circuits. Most electronic goods have noise filters on which cause a small earth leakage. It might be that your "background" leakage is say 20mA and it works fine. But plug in something with an additional 5mA leakage and it suddenly trips. This can unfortunately make fault finding harder!

All that said, I would start with the basics:

1) a visual inspection of all the wiring, switches, control circuits and the motor. Any sign of damage or contamination by water or dirt, oil etc?

2) If you have a multimeter, check the resistance from line and neutral to earth, with the unit switched on(but obviously not connected to the mains!). If there are any electronics inside that will complicate things so you might have to disconnect various bits and test them in isolation. Also make sure it is really switched on. Any no volt release switches will obviously complicated things. Any resistance less 2MOhms is suspicious in my view (although you need to be down to around 10kOhm to cause a trip).

3) call an electrician although be prepared for the electrician to say "the fan is faulty, replace it" so you might be better off with an appliance repair man.

I hope this helps.....

Toby

John Haine06/01/2017 08:05:13
1266 forum posts
86 photos

Could also be caused by a suppressor cap to earth on the fan motor, which might also be leaky.

John Rudd06/01/2017 08:49:49
698 forum posts
21 photos

Take the fan to an approved PAT tester.......

David Jupp06/01/2017 09:30:20
362 forum posts
1 photos

Could be as simple as carbon dust from the brushes creating a path to earth - maybe clean around brush holders etc. Also has the fan been stored for a long time without use? May be damp and need a gentle dry out in a warm place for a few days.

Of course motor could be shot.

John Flack06/01/2017 10:01:06
42 forum posts

Interesting this CU discussion. Last year had to call out a sparkie to change a light bulb which could not be removed

by conventional means. He then gave detailed reasons that our fuse box was not up to code and required replacing, though it had been OK for the 30 years. He was given the job to replace and all was well until I switched some workshop equipment on and was plunged into darkness. He was recalled and fitted an uprated fuse. This appeared to solve that problem, however on Xmas day with the domestic cooking were halted due to loss of power which was located as a tripped fuse unit. The domestic equipment has been the same for the past few years on the old fuse box and worked OK. Why would a new CU make the fuses trip?????

My amateur logic suggests that modern CU s are a touch sensitive to power surge!!!!!!!!!!!!

Toby06/01/2017 10:04:21
101 forum posts
17 photos

A common problem, yes modern RCDs and MCBs are more sensitive and quicker acting than a good old fuse. It just highilghts you had a problem but did not know about it before the board was replaced......

 

Edit: to expand on that, according to my data a 30A rated 3036 fuse (your old fashioned fuse board) will typically run continuously at about 55A and will need 87A to blow in less than 5 seconds.

A 32A type B MCB will take a bit over 45A continously but that rises to 160A to trip in less than 5 seconds.

Edited By Toby on 06/01/2017 10:19:44

anthony smith 406/01/2017 10:17:23
5 forum posts

IF THE FAN HAS NOT BEEN USED FOR SOME TIME MOTOR WINDINGS MAY BE DAMP DRY WITH HAIR DRYER

John Flack06/01/2017 10:17:42
42 forum posts

Thanks toby

But exactly what is the problem that causes the fuse to trip. The concept that I and my good lady have to liaise as to what 'Ration' of the power supply each wishes to use is not my idea of wedded bliss!!!!!!!!!

David Standing 106/01/2017 10:20:04
104 forum posts
8 photos

Does the extractor motor have a capacitor start?

If yes, is it running off a B curve mcb, or a C curve?

John Flack06/01/2017 10:21:59
42 forum posts

Thanks toby

But exactly what is the problem that causes the fuse to trip. The concept that I and my good lady have to liaise as to what 'Ration' of the power supply each wishes to use is not my idea of wedded bliss!!!!!!!!!

Muzzer06/01/2017 10:24:11
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1863 forum posts
318 photos

According to IEE (now called IET) Part P regulations, you are required to fit 30mA RCDs (that term includes RCBOs and SRCDs) to any circuits connected to sockets, so if the problem is due to earth leakage, that needs to be fixed.

There are slower RCDs available but they should not be tripping due to start up surges.

Is this an ancient motor? Some brushed motors take a fantastic surge at turn-on, as the stall current is limited only by the cold winding resistance. If so, you may need either a soft starter or access to a friendly skip.

Toby06/01/2017 10:24:18
101 forum posts
17 photos
Posted by John Flack on 06/01/2017 10:17:42:

Thanks toby

But exactly what is the problem that causes the fuse to trip. The concept that I and my good lady have to liaise as to what 'Ration' of the power supply each wishes to use is not my idea of wedded bliss!!!!!!!!!

John, see my edit to my post above but there are a few reasons why it could be a problem now.

1) as shown above, if you were taking over 45A but less than 55A when cooking the xmas dinner it would be ok on a fuse but not ok on a MCB.

2) If any equipment gives a startup spike of current might not be enough to blow a fuse (they are slow acting) but could be enough to trip an MCB. MCBs have two parts to them, a thermal trip which reacts to long term over current and a magnetic trip that reacts to high currents but acts much faster. Hence any current over 5 times the MCB rating will trip it in less than 0.1 seconds.

3) if it is the RCD tripping now then you could always have had a problem but a normal fuse would not detect that and hence protect against it.

Toby06/01/2017 10:31:28
101 forum posts
17 photos
Posted by Muzzer on 06/01/2017 10:24:11:

According to IEE (now called IET) Part P regulations, you are required to fit 30mA RCDs (that term includes RCBOs and SRCDs) to any circuits connected to sockets, so if the problem is due to earth leakage, that needs to be fixed.

Not quite. I am going to be pedantic here but.....

the IET regulations are BS7671. Part P is a building regulation that does not actually cover the protective devices and wiring requirements.

Les Jones 106/01/2017 10:38:19
1860 forum posts
126 photos

Hi John,
If you read Nick_G's post and Toby's post at 07:58 they both explain the reason for MCB's (Miniature circuit breaker.) and RCD's (Residual current devices.) tripping. A modern consumer unit does not use fuses it uses MCB's instead. Your old consumer unit probably did not have anything to detect earth leakage faults so they could have been present before the consumer unit was replaced. You need to understand that MCB's and RCD's protect against different types of fault.

Les.

Toby06/01/2017 10:46:36
101 forum posts
17 photos
Posted by Toby on 06/01/2017 10:24:18:
Posted by John Flack on 06/01/2017 10:17:42:

Thanks toby

But exactly what is the problem that causes the fuse to trip. The concept that I and my good lady have to liaise as to what 'Ration' of the power supply each wishes to use is not my idea of wedded bliss!!!!!!!!!

John, see my edit to my post above but there are a few reasons why it could be a problem now.

1) as shown above, if you were taking over 45A but less than 55A when cooking the xmas dinner it would be ok on a fuse but not ok on a MCB.

2) If any equipment gives a startup spike of current might not be enough to blow a fuse (they are slow acting) but could be enough to trip an MCB. MCBs have two parts to them, a thermal trip which reacts to long term over current and a magnetic trip that reacts to high currents but acts much faster. Hence any current over 5 times the MCB rating will trip it in less than 0.1 seconds.

3) if it is the RCD tripping now then you could always have had a problem but a normal fuse would not detect that and hence protect against it.

btw, to expand on the domestic strife this might cause. Assuming your problem is just too much load causing an over current trip, your problem is diversity. Most circuits are designed using diversity. This is where the designer will tot up the total load but take into account how much a device might actually be used.

So lets say your cooker is a fancy one with multiple ovens etc. It might be 11kW rated (so that is 47A) . But there is very little chance that all the elements will be on together, and if they are they will not be all on for a long time (the thermstatic control will turn them off when up to temperature). So the guidlines say you should treat it as 10A plus 30% of the remainder, so that would be 10A+12.3A=22.3A in this example. Same with heaters, we take 100% up to 10A plus 50% of that over 10A.

Using these sums we can get a "design current" for the circuit which is much less than the sum of all the rated loads.

The trouble is, if you do happen to use all your machines which the wife is using the cooker to its max and the kids are having showers you can be taking well over the design (and hence protective device) current and cause a trip. This gets worse if (for example) your machines have a high startup current as this will like push the whole system over the edge when you start the machine.

To be honest, I am amazed how little problem this diversity system works. For example in my house I have a 11kW rated oven, an 11.5kW rated shower, 2nd shower, in winter sometimes electric heaters running. Total load well over 150A probably. But it all runs fine of a 60A DNO supply fuse.......

Edited By Toby on 06/01/2017 10:46:59

Toby06/01/2017 10:52:49
101 forum posts
17 photos

Oh, (and with apologies to Robin for going off topic) I was told by a DNO (supplier) engineer that they use diversity when calculating the requirements for new supplies and that they typically set that at 20A per house. So if they are supplying a new development of 20 houses they will allow 400A total for that development.

Obviously individual houses can exceed that (generally the house supply fuse is 100A nowadays) but if the average use goes above 20A per house they have problems!

John Flack06/01/2017 11:14:34
42 forum posts

OK Toby and Nick

I am a dum dum where electrical definitions are concerned , so I employ a professional electrician to solve my problems of ignorence , if I can't use domestic and workshop equipment after paying for his services what are my options to remedy what seems to be a problem???

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