temporary supply for marquee

[ericmark]

As already said answer is generator.

But as to comments about Wellingtons most today have graphite in the rubber mix to stop static shocks so wearing wellies does not stop a route to earth unless specials.

 

[OwainDIYer]

Yep, if you use an extension lead from a PME supply in a garden, the same hazard potentially exists. That's why you should only use 'double insulated' things with such an extension, so that the PME 'earth' is not exposed anywhere.

Which is a very good point, as most people have 'ordinary' (BS1363 socketed) extension leads, which should as a matter of principle be wired with >13A, 3-core flex.

If however these leads are used to run to the garden, there is a danger the earth will be exposed. Even if the appliance is double-insulated, the extension lead socket may have exposed earthed fixing screws.

I bet lots of people run an extension lead out to a 'george forman' type grill for a summer barbecue.

 

[JohnW2]

Yep, if you use an extension lead from a PME supply in a garden, the same hazard potentially exists. That's why you should only use 'double insulated' things with such an extension, so that the PME 'earth' is not exposed anywhere.

Which is a very good point, as most people have 'ordinary' (BS1363 socketed) extension leads, which should as a matter of principle be wired with >13A, 3-core flex.
If however these leads are used to run to the garden, there is a danger the earth will be exposed. Even if the appliance is double-insulated, the extension lead socket may have exposed earthed fixing screws.

Indeed, although the saving grace may be that, IIRC, extenstion leads don't usually have any exposed 'earthed' screws such as you mention. It's obviously a bit of a two-edged sword; having a 3-core cable does mean that an RCD will almost certainly operate if the cable is cut, whilst (depending on circumstances) an MCB will not necessarily operate when a 2-core cable is cut.

I bet lots of people run an extension lead out to a 'george forman' type grill for a summer barbecue.

Certainly. Indeed, can anyone here (many of whom should know better, including me!) put their hand on their heart and say that they've never done it (or worse)?

Kind Regards, John.

 

[holmslaw]

..

 

[Buzx1313]

Marquee.

Metal burco boiler at PME earth potential

Caterer stood on wet ground at true earth potential.

= Injured or dead caterer.

Do not export PME earths for tents / caravans / garden supplies.

Ever.

If you have no hesitation in running out this sort of supply is demonstrates that you do not fully understand electrical principals, earthing supply types, and potential difference.

ok then smart arse, so when you install a socket on the outside of someones house i guess you make that TT then?? no i guess not,, and i suppose the supply to the detached garage will be TT as well, get a grip, if the burco boiler had an earth fault the rcd would trip...

i must remember when i have barby next and i set up my lights i will disconnect the earth from the plug tops and stake them all down with earth various earth stakes dotted around the garden........get real..

 

[Buzx1313]

Nope. The current through your body would have a complicated course. It would start at the L of the supply, flow through one side of the RCD, then through any loads connected to the installation, thence via the N wiring back and through the other side of the RCD, then, when it got to the cutout, it would find no path to the neutral of the supply (broken) - but. instead, would flow along the earthing conductor to the installation's MET and CPCs, along the SWA armour, through your body and then through your wet feet to true earth. The important point being that all of that current would flow equally through both sides of the RCD - hence no current imbalance and no operation of the RCD. You could well be dead.
I doubt it very much, the rcd would have tripped ...

 

[JohnW2]

I doubt it very much, the rcd would have tripped ...

Well, I tried, in good faith. If you don't understand how RCDs work, I wouldn't expect you to be able to understand my explanation. Good luck.

 

[flameport]

if the burco boiler had an earth fault the rcd would trip...

The danger is not from the boiler having an earth fault (that would trip the RCD), but the casing of the boiler having a potential which is different from that of the actual ground the person is standing on. That could occur due to a broken PEN conductor in the supply network, or in some circumstances due to the combination of the various parallel paths back to the supply transformer.

In the event of a broken PEN conductor, an RCD would not trip, as there is no current imbalance between the line and neutral.
The only way to automatically disconnect the supply in the event of such a fault is to use a voltage monitoring device, which would monitor the voltage between the installation earth and a separate earth electrode. Such devices did exist once, but are no longer used for various reasons.

The problem does not occur with outdoor sockets and switches, since they are made of insulated materials. The same applies to most modern equipment designed to be used outdoors - it is typically class 2 and therefore has no earth connection.

 

[Paul_C]

The only way to automatically disconnect the supply in the event of such a fault is to use a voltage monitoring device, which would monitor the voltage between the installation earth and a separate earth electrode. Such devices did exist once, but are no longer used for various reasons.

Now there's a use for those old Crabtree E50 ELCB's when they're removed. Install as a main isolator, and connect the coil as a neutral failure monitor!

 

[ricicle]

i really do not see the problem here,, if you have a supply to a tent or what ever protected by rcds then what is the problem?

I do hope you're not one of the poor souls who are misguided into thinking that RCDs are the be all and end all of electrical safety....?

 

[JohnW2]

I do hope you're not one of the poor souls who are misguided into thinking that RCDs are the be all and end all of electrical safety....?

Well, you can hope .... but all the symptoms appear to be there!

Kind Regards, John.

 

[bernardgreen]

The only way to automatically disconnect the supply in the event of such a fault is to use a voltage monitoring device, which would monitor the voltage between the installation earth and a separate earth electrode. Such devices did exist once, but are no longer used for various reasons.

Now there's a use for those old Crabtree E50 ELCB's when they're removed. Install as a main isolator, and connect the coil as a neutral failure monitor!

Breaking Live and Neutral will not remove the hazard that the PME "earth" wire is at a voltage high rnough above ground to create a hazard to anyone touching the ground and the "protective" "earth".

If the potential on network neutral has risen above ground to a hazardous voltage then the "earth" has also to be disconnected as is is as dangerous as the neutral from which it is derived.

 

[JohnW2]

Breaking Live and Neutral will not remove the hazard that the PME "earth" wire is at a voltage high rnough above ground to create a hazard to anyone touching the ground and the "protective" "earth".
If the potential on network neutral has risen above ground to a hazardous voltage then the "earth" has also to be disconnected as is is as dangerous as the neutral from which it is derived.

Indeed, to protect against high potentials in the network's PEN (due to a break somewhere in the network) would require the disconnection device to interrupt the 'earth' (or the PEN prior to it splitting), neither of which would be permitted.

Of course, in the (fairly unlikely, I would have thought) case of a break in the 'neutral' (actually PEN) between the installation and the nearest PME earhing point, a device disconnecting L would remove the hazard in the installation.

It is true, of course, that if the premises are fully constituted as an equipotential zone, the high potential (relative to true earth) of CPCs and exposed metal would not present a hazard - but a major hazard does exist in the fault situation we are discussing if (per these discussions) the 'earth' is exported outdoors or if there are unbonded or 'unbondable' extraneous-conductive-parts (in the broadest sense) within the premises (e.g. damp walls/floors etc.).

Kind Regards, John.

 

[ban-all-sheds]

A pointless explanation which might as well have been written in mediæval Aleut for all the sense it will make to him.

 

[JohnW2]

In the event of a broken PEN conductor, an RCD would not trip, as there is no current imbalance between the line and neutral.

The problem is that he doesn't understand that, even when explained to him (by a non-electrician ).

Kind Regards, John.