Disconnected Neutal Still Showing Connection To Neutral (somewhere)

[J.A.J.]

hi i am replacing a consumer unit over and came across a strange problem with the neutrals when i isolated the old consumer unit. When isolated from supply im still getting 240v between incoming live and out going neutral. I have disconnected all neutrals from block (which clears it from outgoing at main switch) but all circuit neutrals now show still connected to neutral (somewhere). I was thinking it was the obvious old hookey supply, live chopped away but neutral still connected,but it still dosent make sense. Hope someone here can shed a little light on the subject please. Thanks in advance J.A.J

 

[J.A.J.]

sometimes a N-E fault will stop a RCD from tripping.

What do you mean "stop from tripping"

 

[J.A.J.]

Sorry, with you not mentioning whether or not the existing installation was RCD protected I just assumed that was possibly the reason for the swap of the CCU. Though you may still have a neutral to earth path that doesn't trip the RCD, where any leakage current is less than 30mA.

If there is a current less than 30mA on more than 1 circuit wouldn't this trip the rcd

 

[Midwest]

What do you mean "stop from tripping"

I'm led to believe, if you think of how an RCD works, imbalance between live & neutral, with an earth neutral fault, the fault current in the earth can flow back through the neutral, negating any trip. That's how it was explained to me. But being not expert on the technical bit, however I've had that happen to me.

 

[westward10]

I'm led to believe, if you think of how an RCD works, imbalance between live & neutral, with an earth neutral fault, the fault current in the earth can flow back through the neutral, negating any trip. That's how it was explained to me. But being not expert on the technical bit, however I've had that happen to me.

I believe this can happen on TT systems, for example if you have a neutral/earth fault, the current flow will take the path of least resistance and this is likely to be the neutral path and not earth due to the electrode impedance, hence the rcd will not sense an inbalance to make it trip.

 

[DPG]

I still dont get it. A fault current would still cause an imbalance and trip the rcd. Daz

 

[westward10]

I still dont get it. A fault current would still cause an imbalance and trip the rcd. Daz

Not necessarily if the fault decides to use the neutral path because it is the route of least resistance there will be no imbalance.

 

[DPG]

Ah I see what you mean now on a tt system. Mmm need to think about this one! Daz

 

[telectrix]

i've had it on a TN system. even the test buttons on both RCDs failed to operate them

 

[westward10]

i've had it on a TN system. even the test buttons on both RCDs failed to operate them

It isn't impossible on a TN system but it shouldn't stop the test button operating the device.

 

[telectrix]

i know, weird. i was on it for over an hour before finding a radial the used to feed the immersion heater but was dissed at the load end was the cause.

 

[westward10]

i know, weird. i was on it for over an hour before finding a radial the used to feed the immersion heater but was dissed at the load end was the cause.

Must admit have never experienced this just know the supposed theory behind it so I am just guessing with the test button. Weird stuff this lectricity.

 

[Midwest]

i know, weird. i was on it for over an hour before finding a radial the used to feed the immersion heater but was dissed at the load end was the cause.

Ditto same for me, rewire on a TN-S. Found fault N-E on a radial, thought put that to one side while I connect the rest up, except I mixed up the faulty one with another and connected it up. All went well until went to test RCD, which timed out. Test button didn't work either. Took a while before I realised mistake.

 

[R G]

Is the supply PME by any chance , this situation is something I experienced working for a DNO , where a reverse polarity or broken neutral even externally in an underground joint meant that the neutral became live and returned a live supply via the earth and bonding connections from neighbouring properties .....

 

[Wilko]

RG gets my ho-lee-cr@p award for the day with that gem.

 

[J.A.J.]

Is the supply PME by any chance , this situation is something I experienced working for a DNO , where a reverse polarity or broken neutral even externally in an underground joint meant that the neutral became live and returned a live supply via the earth and bonding connections from neighbouring properties .....

Hi ,it's a TNS system ,checked the earth coming of cable head (ze 0.30ohms) no return voltage between incoming neutral and earth. It's an old cast 3 fused cable head which also had me thinking about the neighbour scenario but no other cables are coming of it . Had to put the CU back together as had to leave the job cos the customer was going away this morning