Whatif - Lost supply neutral?

[-Sean-]

DNO visited this week to move tails to new CU from old breaker. When he pulled the main fuse I noticed there looked like two ceramic carriers inside, I asked and he confirmed both live and neutral were fused. It's being fed back, don't know if they'll do anything about it.

In the meantime is there any risk if the neutral side fuse were to blow?
Will the RCD still protect in the same way?

System is TT, split load, 100mA time delay protects whole installation, 30mA for sockets etc. Designed to 16th (started months ago!)

 

[securespark]

Most likely neutral is a solid link. What do you mean by fed back?

Ask for PME supply, replace the S type with a 30mA G type RCD and fit a main isolator in the tails and hey presto, you have a 17th setup...

 

[Spark123]

With a fuse in the neutral, a TT is probably safer at the mo!
Not quite as easy as that ss
Lights etc will be on one RCD, all the sockets etc on the other. One RCD will be feeding the other. But I get the essence of it.

 

[securespark]

Sorry. It's late.

Forgot to mention you'd need to reconfigure internal wiring in the split CU... and then add a henley outside to split the feed to both RCD's independently.....forget it.

Please disregard my ill-thought out advice.

 

[-Sean-]

By 'fed back' I guess the guy from the DNO will mention it to the office, I don't know if that means some follow on work from them or not.

I enquired about PME before, they want £100 to do it, I'd have to change the main bonds for meatier cable of course, and rethink the disconnection times for the non-RCD circuits. Easier to comply when everything's on an RCD!

Had the forsight to put the 100A isolator that was spare from the CSU (swapped for the 100ma time delay) in another enclosure with the tails thru it, that way I can isolate the CSU if upgrades to 17th required later on.

Thought about 17th, however all lighting is on one circuit (existing one reused), I guess to comply I'd have to provide some discrimination of upstairs and downstairs - that would mean some re-plastering I'd rather avoid!

It could be the neutral fuse carrier is just a link, I didn't think to ask him to have a look.

Anyway, back to the safety aspect, if the supply neutral fuse did blow, I guess direct/indirect contact with live would still cause the RCD to trip?

 

[Spark123]

As you say your earthing is TT then the loss of the neutral will not cause great danger - OK everything will stop working but that is all which should happen. One point to note is that there will be 240v on the neutrals within the installation with respect to earth. All exposed and extraneous conductive parts will still be bonded to earth by the earth electrode.

If you were to have direct contact with a neutral conductor whilst it is at 240v then you will recieve a shock, in the case of a 30mA device it should still trip before you recieve a fatal shock. In the case of the 100mA time delayed device you may recieve a fatal electric shock as it is not designed to provide supplementary protection against direct contact. Any earth leakage above the settings of the RCDs should trip them.
To have direct contact with the neutral you will probably have to do something deliberate i.e. remove a cover.

It is more dangerous where TN-CS (PME) is involved as all exposed conductive parts and extraneous conductive parts are bonded to the neutral conductor of the supply. If this was to go open circuit then all exposed and extraneous conductive parts will also become live to 240v with respect to earth.

 

[IJWS15]

I thought Neutral earths were banned - at least they were in the 14th ed. when I last re-wired a house.

50s wiring with live and neutral fuses on each circuit (no rings) and a large cast iron on-off switch under the stairs with main live and neutral fuses in it (90 amps cpmprising 3 strands of 30 amp fuise wire).

Was enough for the DNO to disconnect then.

Problem is everyone thinks the power is off but it isn't.

 

[Spark123]

A consumer is not allowed to combine the funtion of a neutral and earth in the same conductor.

TN-CS is a supply type where the neutral and earth are combined in a PEN conductor up until the service head. The supply is technically a TN-C up until the service head where it is separated out to two terminals, one for the consumer neutral and another for the means of earthing for the consumer.
The TN-CS side of the system is owned and operated by the DNO - not the consumer.

 

[ricicle]

If you were to have direct contact with a neutral conductor whilst it is at 240v then you will recieve a shock, in the case of a 30mA device it should still trip before you recieve a fatal shock.

As long as that Neutral wasn't on the supply side of the RCD (from the load)

 

[plugwash]

I thought Neutral earths were banned - at least they were in the 14th ed. when I last re-wired a house.

They are banned from normal installation wiring but allowed in suppliers wiring under certain conditions (PME) and also allowed in certain other situations that most sparkies will never deal with.

 

[IJWS15]

Apologies - Meant to say Neutral Fuses.