neutral as an earth

[whiz]

In PME, neutral is connected to earth. Why can't you use neutral to provide earthing in domestic applications, ie, earthing metal socket boxes with a link to the neutral? Why is there a need for a separate protective circuit?

 

[Millennium_Boy]

Because BS7671:2001 wiring regulations say you must, and so does the Electrical safety, quality and continuity regulations 2002.

 

[Adam_151]

Imagine a broken neutral, someone wired a a light switch on neutral by mistake, now imagine yourself standing on the earth and touching the the metal casing of your light fitting (which is connected to neutral) as you change a bulb...

 

[whiz]

I understand that the regs require this, and its not a question of doing this as an alternative to this requirement - I wouldn't even contemplate it! Its more out a question out of curiosity if any! So assuming the installation had been wired correctly, is there any reason in theory why it couldn't? Someone has said that you can't guarantee that the neutral is a O potential, is this true!

 

[plugwash]

its known of as a TN-C system but its banned in the uk at present

i belive it was allowed in the past but the cableing requirements for it were very strict (ie earth/neutral surrounding the live)

personally i am against the whole idea of pme and belive that regs should insist all new installs by recs are TN-S unless existing cable runs make this impractical

 

[algenon_iii]

I understand that the regs require this, and its not a question of doing this as an alternative to this requirement - I wouldn't even contemplate it! Its more out a question out of curiosity if any! So assuming the installation had been wired correctly, is there any reason in theory why it couldn't? Someone has said that you can't guarantee that the neutral is a O potential, is this true!

Neutral (Return) not being at 0v is true. The guy who (many moons ago) taught me elctrical engineering said there was just about enough P.D. between earth and netural to drive a door bell (and told us not to try it at home).

 

[plugwash]

NE voltage varties quite a bit but should really never be outside the ELV range unless there is a broken neutral (50V of volt drop on one side of the supply would be pretty insane)

i have personally done a lot of wiring with only the live isolated and have never had a shock off a neutral (NE faults will cause enough current to trip a rcd though which is a pita)

a TN-C system wired completely in single core pyro with special precautions at the glands would be safe and there are mentions of this in the IEE regs iirc although it is presently banned by the ESQCR and the IEE regs currently have to reflect this

fact is though except in huge power stuff wiring to the specs needed to make TN-C safe would be prohibitively expensive (pyro doesn't come cheap)

also unless you fancy splashing out a fortune on rcd sockets rcd protection would be a real problem in a TN-C install

 

[ZenStalinist]

Why can't you do that in the UK?

If you look at what the Americans used to do (TN-C, and tying equipment chasis off to neutral), it answers your question. Live/neutral reversals and broken neutrals frequently resulted in people being electrocuted. Another problem is that when this was introduced along with polarized plugs and sockets, a lot of people ignored the advice in the manual, and went on to stick a nonpolarized plug on because their house hadn't been rewired. This results in 50% chance of a hot chasis. TN-C installs are still occaisionally found over there, where a two pin system has been converted to three pin sockets. But then USA is the same country that allows you to wire a grounded outlet into an ungrounded circuit, as long as it has RCD.
As knowing quite a lot about electronics, I have found it still, despite grounded outlets, very common practice to tie the neutral off at the chasis and leave it at that (it is allowed in many circumstances as opposed to grounding). Of course, us limeys know better...