N/E fault RCD not tripping ??

[Buzz1980]

Hi all

can anyone give me the logic why the RCD wouldn’t trip on a neutral/earth fault. Testing a extension where they have added in 5 additional sockets to the upstairs ring had been powered up and nothing tripping with household appliances plugged in and working. Carrying out IR tests to find had dead short on one leg of the neutral and earth between 2 sockets but there was no issue of tripping ? RCD checks out ok with test button and RCD tests. Have disconnected this leg and then fitted fuse spur off the ring to carry the additional sockets now as to late to add in additional cable, so extension spurred off ring using fuse spur. Would it be the case that it had seized a bit RCD or that its connected to a TNCS system?? Replies much appreciated ?

 

[Grant1987]

250volts to see if ok then 500volts after that test ?

Ok well where did you get the >999Mohms?

 

[Wilko]

I don’t think so as I managed to trace the fault and link between two sockets N to E short. But that may be a case in future works.?

Oops, sorry I missed that

 

[Buzz1980]

On both tests at 250v and 500v. I tested live to earth at 250volt seen that it was very low near enough zero, now this has been connected to a RCD and MCB for years this circuit. So I then joined live and neutrals together at consumer unit then tested live to earth IR test at nearest socket comes up >999mega ohms. So I am thinking I’m testing through the USB points when carrying out live to earth only with neutrals still connected to N bar ?? That make sense

 

[Grant1987]

Ok well where did you get the >999Mohms?

Googling it now are you? Lol

 

[Buzz1980]

Sorry googling what ???

 

[Grant1987]

On both tests at 250v and 500v. I tested live to earth at 250volt seen that it was very low near enough zero, now this has been connected to a RCD and MCB for years this circuit. So I then joined live and neutrals together at consumer unit then tested live to earth IR test at nearest socket comes up >999mega ohms. So I am thinking I’m testing through the USB points when carrying out live to earth only with neutrals still connected to N bar ?? That make sense

Testing at 250v is only beneficial when there is a fault so that you don’t need to subject circuit to 500v however you need to carry out insulation resistance test at 500v to record accurate fault reading and true scale of insulation. I’m trying to test you here ?

 

[Grant1987]

Testing at 250v is only beneficial when there is a fault so that you don’t need to subject circuit to 500v however you need to carry out insulation resistance test at 500v to record accurate fault reading and true scale of insulation. I’m trying to test you here ?

I’m here to learn just like yourself and Aslong as it’s done appropriately then there is no harm. Best way is to ensure everything is unplugged from sockets

 

[Buzz1980]

Yeah appreciate the response, but it has just got me thinking why it will show up dead short L- E but then join neutral and live together, IR test at nearest socket and comes up clear ?

 

[Grant1987]

Yeah appreciate the response, but it has just got me thinking why it will show up dead short L- E but then join neutral and live together, IR test at nearest socket and comes up clear ?

Doesn’t sound right to me butty, needs further investigation without a doubt

 

[Grant1987]

Doesn’t sound right to me butty, needs further investigation without a doubt

If it’s dead short L-E should be a banger? Is it old consumer?

 

[Grant1987]

If it’s dead short L-E should be a banger? Is it old consumer?

These are the jobs I hate, because you have to explain to customer that it’s not right even though it appears fine and well. Don’t leave it there find the fault and charge accordingly you’re the last sparky there so when you close that door it’s your neck on the block

 

[Buzz1980]

No it’s 6 months old, that’s why I am thinking I’m testing through the USB ports only thing I can think of as if it was a dead short then it would show up on L- N together and test to earth as well but it’s clear ?

 

[Grant1987]

No it’s 6 months old, that’s why I am thinking I’m testing through the USB ports only thing I can think of as if it was a dead short then it would show up on L- N together and test to earth as well but it’s clear ?

Well disconnect the usb sockets put the cables in connector blocks then test. Make sure you are disconnecting the circuit in question out of bar/mcb to ensure it’s definitive. I’m a bit confused here sorry it’s late and been working 14 hours again ?‍ You’ve been called back after 6 months and it’s tripping?

 

[Grant1987]

Well disconnect the usb sockets put the cables in connector blocks then test. Make sure you are disconnecting the circuit in question out of bar/mcb to ensure it’s definitive. I’m a bit confused here sorry it’s late and been working 14 hours again ?‍ You’ve been called back after 6 months and it’s tripping?

The USB ports will 100% interfere with you’re readings and give false readings disconnect them. So the fault is on the sockets? Unplug everything and then carry out insulation resistance. Once established that the fault is on the ring main. Most likely pain in the arse is N-E fault split the legs at consumer go to half way point of ring (take an educated guess) then you will know which side the fault is on)

 

[Buzz1980]

Sorry no the consumer unit is 6months old, this was a extension that got done and they require test cert. yeah think best to disconnect but then test kind of pain in the arse tbh is it disconnecting circuits ?