I have just been involved in a situation where temporary loss of neutral on a TNCS system caused thousands of pounds worth of damage. It seems this loss of neutral situation, either within or outside the installation, is occurring more frequently. SPDs are now commonly fitted but at DBS and generally with a Up in the order of 860v, so giving no protection on loss of neutral in a three-phase and neutral system. Cost benefit analysis across the national spectrum might not support a compulsion but is it time designers should be raising the issue with clients and at least offering a solution. On the other hand, is there a packaged solution?
But would it not stop diverted neutral currents flowing in my bonding?
If I've pictured it correctly, L & N are disconnected, as are main bonds - but the main earthing conductor is still connected to the DNO's earth terminal - and hence the c.p.c.s to any equipment that happens to be in contact with metallic pipework (boilers, pumps and so on). So the diverted N currents will still flow, just now all through 1.0mm² or even 0.75mm² c.p.c.s rather than mostly through 10mm² bonding conductors. Given that diverted N currents (especially in a broken PEN situation) can be many tens possibly even hundreds of amps, I think I'd prefer to keep the 10mm² connected...
- Andy.
Maybe I phrased it poorly ... I was thinking of the MET & everything connected to it (bonding, circuit cpcs etc) being disconnected.
I agree with your point... dropping the bonding but leaving the 0.75mm^2 cpc of the central heating pump etc connected would jjust make the situation even worse. I'm envisaging complete disconnection from the DNO network.... L, N and main earthing conductor.
But would it not stop diverted neutral currents flowing in my bonding?
If I've pictured it correctly, L & N are disconnected, as are main bonds - but the main earthing conductor is still connected to the DNO's earth terminal - and hence the c.p.c.s to any equipment that happens to be in contact with metallic pipework (boilers, pumps and so on). So the diverted N currents will still flow, just now all through 1.0mm² or even 0.75mm² c.p.c.s rather than mostly through 10mm² bonding conductors. Given that diverted N currents (especially in a broken PEN situation) can be many tens possibly even hundreds of amps, I think I'd prefer to keep the 10mm² connected...
- Andy.
Maybe I phrased it poorly ... I was thinking of the MET & everything connected to it (bonding, circuit cpcs etc) being disconnected.
I agree with your point... dropping the bonding but leaving the 0.75mm^2 cpc of the central heating pump etc connected would jjust make the situation even worse. I'm envisaging complete disconnection from the DNO network.... L, N and main earthing conductor.
