Hiya Guys,
Nothing like....MEN....urrrgggghhhh I used to hate talking about this, but I'll see how I go..........maybe another beer would be in order first...
Please also be aware that as this is only my second post here......all corrections will be appreciated.
Yes, open circuit neutrals kill plenty of people, every year all contractors in Victoria ( used to be my home state ) would get sent the *death list*.It never made pleasant reading either. Plumbers usually bore the brunt of undetected neutral faults, I think one year a landscape gardener even died whilst carrying out some unlicensed plumbing work.
Other more common problems would be house fires from earth conductor overload, more often than not it would be the house down the road rather than the house that had the O/C neutral.
The warning signs nearly always being *that* call from a customer "I'm getting shocks when I touch my taps".
As voltage rise on the earthing system is load dependant some faults would go undetected for ages. The design/nature of the MEN system also used to make it real hard to pinpoint the fault location as more often than not it would be on someone elses installation.In one case I do remember the supply authority ended up checking every house in the street...and then found the fault was at their own tranny two streets away.
As for the neutral / earth connection...well from what I can remember ( excuse the rusty brain ) the key word is *integrity*.
The disconnection of the main earth from the neutral bar or link has to be able to be carried out in a manner that does not destroy the integrity of the installations earthing system.
In an older style board ( neutral bar only ) this was pretty easy to achieve. All the circuit earths were soldered on to the main earth, the main earth could then be taken on and off the neutral bar as many times as you liked and earthing system integrity was maintained.
Things got complicated when earth bars were introduced into domestic switchboards...
Only kidding about that.....but the number of guys that used to get confused about this was pretty amazing. It never mattered which bar you went to first with the main earth as long as earthing system integrity was maintained. A lot of guys would get caught out (defected) because they would twist terminate into the neutral bar first then jump to the earth bar. The twisted up earth in the nuetral bar (unless soldered) could never be considered as having integrity once it was removed.
With regard to the "missing MEN link" what has be considered is the type of fault or overload condition.
Straight phase to neutral overload it doesn't really matter if the links there or not.
For a phase to earth fault you really have to consider the fault current path.......
Now this is were I might stuff up, ( another beer maybe.....) strictly from memory we used to be taught that under an earth fault condition approx 90 - 95 % of the fault current would use the main neutral conductor from the MSB onwards, the balance finding it's way back via the installations main earth generally through some one elses switchboard or sometimes directly to the sub or tranny depending on its proximity.
With the MEN link removed the fault current usually has no other choice than to go to ground and usually still looks for a neutral to get back on ( electricity is so lazy it never wants to work hard and always looks for the easiest path...
)
What pauluk has said with with regard to lack of fault current to operate protective devices is correct in theory and can possibly happen ( particularly in areas with high ground resistance and isolated installations) but in an MEN system generally in practice fault current always seems to catch a free ride on someone elses neutral.
A single installation could have it's link out for years and even with faults nobody would be non the wiser.
No MEN link also of course means the neutral has potential for voltage rise and is floating around all over the place...which can also be interesting......
The voltage operated ELCB's used to be mandatory in rural installation areas that were suplied on the older style 11Kv SWER ( Single Wire Earth Return ) systems. Not quite sure about nowadays though.
ciao