High zs tripping mcb?

[thekingiam]

First of all readings from kitchen ring circuit are as i remember insulation resistance all well above 2 meg now!problem i had was extension had flat roof and cable was damaged between two points replaced damaged cable but still mcb not rcd as they aint got one is breaking.this was when no load present and thought rectified well it lasted 3 days then phone call that it had tripped again went back today took a couple of readings before forced out on other job what i took was zs readings from all sockets and got average 0.34 then a reading at replaced cable socket say socket A of 47 ohms tried reading at connecting socket say socket B in ring and got 46 ohms, disconnected new earth from first socket A and A read at 0.45 and B read at 347 ohms read all sockets and they were still normal.

so i have deduced that a bad earth connection from socket B to a socket before and will test continuity on sunday but my main question is how a large zs can cause an overload to trip mcb?
or after solving this problem should i look further?

 

[GLENNSPARK]

OK well for a start overload would be Zpn...wouldn`t it...

 

[Marvo]

....how a large zs can cause an overload to trip mcb?

High Zs would cause disconnection times to be longer than allowed. I don't fully understand your post TBH but high Zs won't cause an MCB to trip.

 

[trev]

Think about what an MCB is mate.
As Marvo said, a high Zs wouldn't cause the tripping. What were your original continuity readings?
Has there been any buggering about with the circuit between when you were last there to now?

 

[thekingiam]

well thats what i thought but why if disconnect 1 earth in ring ...... ok thinking .....if neutral connected to earth at supply and neutral live leakage.........no stop ! ime stumped lol.
maybe i need to re check insulation resistance as i think whatever problem was there has now moved???? i need a drink.

 

[thekingiam]

to be honest trev i went in bad ins res readings live neutral tracked it down replaced cable no problem and left, i know i should of tested further but one of those jobs when some indian electrician over my shoulder trying to tell me that he thinks its just a faulty socket and hagling price to next to nothing for me.

 

[NickD]

I'm struggling to understand your meaning a bit as well, mate, to be honest...but yeah the mere fact of a high Zs in itself cannot as far as I can see cause an MCB trip. Unless it was something like the cause of the high Zs was actually a high R1, i.e. line connection is offering an impedance, which will cause conductor heating and expansion/deflection, which could make it touch something it shouldn't be touching...wild speculation really. I'd suggest just identify and fix whatever the problem is that's giving you those godawful high Zs readings and then see if the problem goes away. Something is deeply b*gg*red there as it stands; maybe once you find exactly what and how it is, it will shed light.

 

[thekingiam]

High Zs would cause disconnection times to be longer than allowed. I don't fully understand your post TBH but high Zs won't cause an MCB to trip.

well i thought that myself and thats why i posted and i am glad my theory was correct but when looking today found another fault and was locked into that fault, only now later i am thinking after a beer.

 

[ferg]

well i thought that myself and thats why i posted and i am glad my theory was correct but when looking today found another fault and was locked into that fault, only now later i am thinking after a beer.

Easily done hear hooves, think horse, rule out horse must be Zebras.

Really it's just another load of horses

 

[thekingiam]

thanks nickd but as always no customers and now ime overloaded i feel for some customers but i do expect to do half day testing and rectification for if ime lucky 50 quid off these, my problem is if i take a payment for rectification no one wants to pay for further works.

 

[Richard Burns]

MCB trips because of excess current flow through a live conductor (though it may be going to earth) .
You have measured Zs and got a high reading that was OK after removing the new earth cable, so there had been a loose connection on that earth cable.
If that is now resolved then you are looking for a new fault (not the original damaged cable, unless you damaged the new cable!) that is causing the MCB to trip.
You say that there was no load on the circuit, was this because everything had been disconnected / unplugged and all FCUs unwired or was it because everything was off. If the latter then there could be a faulty switch to a faulty appliance or a fault on an appliance on an unswitched FCU.
If the former then you still have a circuit fault, presumably intermittent, so it is work round the ring looking for errors somewhere. repeating your IR tests at each point in case something shows up.