MCB tripping at random times

[ban-all-sheds]

An hour?

It's hard to read the time/current curves at that sort of time, but as far as I can see you'd need 45-50A for that, 11kW-ish. That's a lot of heat generated in an hour - think how long a 2-3kW kettle takes to boil a litre of water.

But there is clearly something wrong under that tile, so it needs investigating.

160A to trip one? Wouldn't that take out the company fuse?

Oh no - an 80A service fuse will let 160A through for about 20 minutes.

http://www.google.co.uk/search?hl=e...ime+current+curves&btnG=Search&meta=&aq=f&oq=

 

[Beelzebub666]

Thank you for the help.

These two cables are the new colours, I guess put in with the extension as a ring hooked into a junction box somewhere with the single cable back to the CU.

As part of my prior investigations i found the first box on the new ring (which is after the suspected fault), and i unhooked it killing the rest of the ring but not stopping it tripping. So if I cut in before it goes through the wall and route a temporary cable above ground to replace the suspect section of the ring, that'll prove if this is the problem or not without breaking out the tile. If I can confirm the problem it'll save her a lot if she gets someone in to do the work.

 

[ban-all-sheds]

As part of my prior investigations i found the first box on the new ring (which is after the suspected fault), and i unhooked it killing the rest of the ring but not stopping it tripping.

That's because it's a ring - if you break it in one place all of it is still live.

But you've then got 2 radials on a breaker which is too big for 2.5mm² cable.

So if I cut in before it goes through the wall and route a temporary cable above ground to replace the suspect section of the ring, that'll prove if this is the problem or not without breaking out the tile.

Yup, if whichever sockets etc on that section can be lived without for a while.

If I can confirm the problem it'll save her a lot if she gets someone in to do the work.

Ah.

 

[ericmark]

I do remember one job where we ran out of cable joints and until they arrived only option was to use denso tape. Careful note was taken for each repair and once the cable joints arrived the cable was dug up again and proper joint fitted.

Around two years latter I found the new electrical engineer scratching his head in generator shed as we where generating around 750KW but he could only account for 400KW and was looking for another feed to something as yet not found.

I told him of earlier problem but if someone had done a repair without making a note it would not be easy to find.

However our luck was in as it snowed that night and in the morning we could see from melted snow where the bad joint was.

On digging up we were surprised to see how well it had lasted and how little way down the cable had been damaged. Two joints were installed and we were able to reduce the number of generators.

However it did point out how much power can be absorbed into the ground.

In this country of course denso tapes joints under ground would likely find one in court but in the Falklands we got away with it. To have found who had done it would have been near impossible although I have a good idea who it was.

Before anyone shouts at me I personally did not do any denso tape joints.

 

[Beelzebub666]

As part of my prior investigations i found the first box on the new ring (which is after the suspected fault), and i unhooked it killing the rest of the ring but not stopping it tripping.

That's because it's a ring - if you break it in one place all of it is still live.

I was assuming it was a ring because i saw two cables, but when I took off the sockets and unhooked the wires it turned out that it's only live at one end, the second cable is actually just a feed for two sockets in the kitchen. I should have realised when unhooking that one socket did kill the rest of the 'ring'.

Does that mean the whole thing needs to be condemned anyway? Should I tell her she's been living in a deathtrap for years and is lucky to be alive?

 

[ban-all-sheds]

I think that's a bit extreme - 2 sockets on a spur is not allowed, but if done in 2.5mm² it's unlikely to be a death trap.

And as for my earlier comments, and having read what Eric said, I hadn't been thinking properly - I was picturing a L-cpc or L-N fault, but I wonder if it could be a fault to actual earth, i.e. not the cpc?

What think we chaps? It would mean the equivalent of a TT Ze of <5&#937; to trip a B32in about an hour.

 

[Beelzebub666]

I think that's a bit extreme - 2 sockets on a spur is not allowed, but if done in 2.5mm² it's unlikely to be a death trap.

And as for my earlier comments, and having read what Eric said, I hadn't been thinking properly - I was picturing a L-cpc or L-N fault, but I wonder if it could be a fault to actual earth, i.e. not the cpc?

What think we chaps? It would mean the equivalent of a TT Ze of <5&#937; to trip a B32in about an hour.

What i mean is, there is no ring, the whole circuit is hooked up like a lighting circuit with one 2.5 cable out of the CU, it has the two sockets in the kitchen, two in the hall with a fridge, tumbledryer and washingmachine, and spurred off it is a fused light, and the boiler and a towel rail. It must have been like that for years now, but it does seem a little dodgy. If the last socket in the kitchen is the end of the mess, I guess I can just take that back to the CU to make a ring.

I figured it must be a L-E arc when you pointed out it would otherwise be very effective underfloor heating.

 

[ericmark]

Looking at Figure 3.4 page 249 seems we could be looking at around the 45A mark but thinking about showers where some one has put a 10.5Kw shower on a 32A MCB they trip in a lot less than one hour so I would say around the 40A.

Now a 2.5mm cable at 40A would really get hot and the electric meter would be going round like something daft so although a 5 ohms earth connection could cause that current I would have thought if that were the case

"Beelzebub666" would have noticed the very hot wires.
So I think we are still looking at a very heavy short for short time not a 40A drain over an extended time.

The voltage under the floor does point in that direction and I suppose each time it grows a carbon track the current could then blow it clear again.

With this sort of fault where it seems the cable is overloaded anyway one could very easy have a fire and I would want to isolate until cured or at least fit correct size MCB for cable and even if temporary I would be fitting 20A MCB.

On motor vehicles I have wired faulty circuits through a large lamp i.e. head lamp and then waggled all cables until the lamp lit but such an approach with 230v would be very dangerous and as a result it can take ages to find a fault.

So in 230v circuits we need to use the insulation tester and the only option is to test dead. Although the insulation tester can give one a nasty belt it is far less likely to kill anyone and you can still waggle wires or stamp on floor with mega running in same way as bulb with motor vehicle.

But the mega (insulation tester) is not cheap. Never tried to hire one but I expect you can? But as I have said even a mega can give you a nasty belt as it sends it's 200ma through the body.

So "Beelzebub666" has to decide if he should continue or if this it the point where he should say "Sorry this job needs someone with proper test equipment?" especially considering the serious nature of the faults suspected.

A few months ago we were reading about a lady who was killed after her husband though there was something wrong and contacted the land lady would in turn got hold of an electrician but before he could fit the job in she died.

The Electrician, Land Lady, and Husband must have all blamed them selves and though "If only I had" and from what we are told this could so easy be another "If only I had" and personally I think it's time to get in someone who does not need to ask on here what to do.

 

[Adam_151]

? But as I have said even a mega can give you a nasty belt as it sends it's 200ma through the body.

If it did that that then it wold be a possitivly dangerous instrument ... you are thinking of your low ohms meter; that puts out 200mA at 24v. Your megger on the other hand is limited to 1mA ... afterall if you do ohms law thats all you need to be able to sustain 500v in a cable thats down to half a meg

 

[ban-all-sheds]

If it did that that then it wold be a possitivly dangerous instrument

And being an electrical apprentice would be one of the most dangerous jobs in the world.

 

[ericmark]

You are right 200ma is for low ohm reading for insulation tests lowest pass mark is 250k ohms so it has to be able to maintain 500 volt into 250k so 2ma as to if able to kill I don't know. Megger has 1ma at test voltage and 1.5ma at short circuit.

As an apprentice I went to another depot and noticed the mega connected to vice. Leaning on the vice I was approached and as I saw his finger go to button I put fingers of my other hand on his neck. Thought I would teach him a lesson.

Well he fell to the floor and for a few moments I though I had killed him. The lesson was learn by us both. I never tried to give anyone a shock after that.

I would guess he was not in best of health to start with but still remains a possibility that although 99 times out of 100 one can stand the output from an insulation tester that does not mean there is no danger.

It seems academics can't agree

Zitzewitz, Paul W., Neff, Robert F. Merrill Physics, Principles and Problems. New York: Glencoe McGraw-Hill, 1995.

"The damage caused by electric shock depends on the current flowing through the body -- 1 mA can be felt; 5 mA is painful. Above 15 mA, a person loses muscle control, and 70 mA can be fatal."

But I think I would strive to not get any electric shock other than may be from TENS Machine.

 

[Adam_151]

lowest pass mark is 250k ohms so it has to be able to maintain 500 volt into 250k so 2ma as to if able to kill I don't know. Megger has 1ma at test voltage and 1.5ma at short circuit.

That would be for ELV circuits which are tested at 250v though... still works with 1ma

 

[Beelzebub666]

Alrighty, I downgraded the circuit to a 20A MCB, and disconnected everything after the first outlet which includes the bizarre live floor tile adhesive. The boiler is hooked in from that first outlet, protected by a 3A fuse, so she at least has heat, and there's still one outlet in the kitchen which is on the cooker isolation box on a seperate circuit. I explained I wasn't willing to reconnect the rest of it on a spur even bypassing the underfloor dodgy bit, and she's going to get some quotes on rewiring the lot by a pro. I wrote 'installed by Eric Mark' on the CU, just in case.

I suspect a paleo-circuitologist would find that what has happened is that fuse originially just did a water heater, and over the years various cowboys have spurred off it again and again building up to what really needs to be a ring main.

 

[ericmark]

I have forgot that items with high earth leakage had been changed in 17th Edition but even 3.5 ma is 65K&#937; so I would have though one should be able to measure that at 500v but it seems not.

However main thing "Beelzebub666" has realised the danger and acted on it. It is true that over time alteration upon alteration can result in wiring which non of the people who installed it would have ever knowingly done.

We have all seen where it's only for TV and Video so why not fit 4 sockets. Then some one else says there are two cables here must be part of ring.

Even my Dad's house which has only had one owner since 1954 still has pipes and wires which no one can recall what they are for or who fitted them.

I do hope it is all sorted soon. But at least in mean time you are not walking on egg shells. And you can sleep at night.