Brewing Shed advice

[beermonsta]

I'm with Dr. Dextrin and Boingy on this one.
"but without a connection to earth at the point of supply there's still no path for fault currents" Of course there is a path to earth, through the poor bugger who is being electrocuted, i.e. that person becomes the earth.

The RCD will monitor the balance between the live and neutral, and as soon as the current leaks to earth through the person (being frazzled) it notices an inbalance and therefore trips. It doesn't monitor the earth at the point of supply so it's completely irrelevent if it has high impedance or doesn't even exist at all.

[themadhippy]

For a current to flow you need a complete circuit.

excuse the drawings,im no artist.in the top picture the fault current travels through the body to ground,but as there's no earth the circuits incomplete,so no current can flow.In the bottom picture there is a connection to earth at the point of supply the circuit is complete so a fault current can flow.

[Kev888]

For a current to flow you need a complete circuit. excuse the drawings,im no artist.in the top picture the fault current travels through the body to ground,but as there's no earth the circuits incomplete,so no current can flow.In the bottom picture there is a connection to earth at the point of supply the circuit is complete so a fault current can flow.

Hi,

I'm afraid you're getting a bit confused here - as you stated earlier there IS a path for the current back to the source because the whole supply is not floating - and if there wasn't then the stick person would probably be safe anyway! The flow through the earth doesn't have to return specifically to the fuse box or down an earth wire, it can go all the way to the supply company's nearest earth rod or supply station. Just as long as it is sufficient to draw over 30mA the RCD will still trip because it looks for an imballance in current between live and neutral; if there is one then clearly something else outside of these intended paths has become part of the circuit back to the supply. It doesn't care if its travelling down the protective/earth wire or through my feet (neither of which is great)

Some domestic supplies (PME) have the earth and neutral directly connected - there are bigger requirements for primary earthing with these. Other supplies are less formally associated with earth, but the neutral is still 'about' at the same potential as earth. In both cases this puts Live at 'about' 240v above earth, so there is about as much potential for a fault to cause current flowing to the earth wire as there is to the ground through (e.g.) the brewer. The only way to get around that is to use an isolated floating supply, such as from an isolation transformer or by brewing suspended from a helecopter.

If the brewer is isolated enough from earth (e.g. insulating soles, on a wooden step ladder etc) and manages somehow to bridge live and neutral then 30mA may not escape to earth and the RCD will not trip. RCDs are not perfect for that reason, and the fuse or MCB is also extremely unlikely to go, because the resistance of the human body won't easily permit enough current to flow to activate over-current devices. In that situation having the appliance earthed certainly reduces the chances of a flailing brewer not somehow contacting earth, but as it only takes 30mA to trip the RCD then to be honest whilst a good earth is better than a bad one I'd struggle to imagine a 1.5mm earth cable of any normal length inhibiting that if the soil itself doesn't.

Cheers
kev

[boingy]

This has got to the point where I'm starting to think it is a wind up so, to quote a Dragon, "I'm out".

But Kev, I think the brewing from a helicopter idea could be a go-er. Definitely worth some exploration that one, although the hourly running costs might be a bit prohibitive...

[Kev888]

Hmm, you'd imagine there'd be less of a condensation problem too with all that down draft. I wonder if helecopters come in stainless..

[themadhippy]

Kev the main pointy i was making was,without an earth point then an rcd would not work.as an aside ive seen an installation were the metal frame of a greenhouse was live,touching it resulted in a shock,but the rcd in the consumer unit didnt trip,but did operate correctly when tested within the house,no earth and a damaged cable was the cause.showing that mother earth aint that good a conductor.

[beermonsta]

Ha Ha - I'm still in Boingy, as I'd like to see this one to the end!
Your diagram madhippy fails to demonstrate that earth (aptly named) is as Kev says a potential that is pretty much at the same level in the ground as in the "earth" that is possibly supplied by your electricity company. By all purposes they are all one and the same. So it doesn't matter if there is no connection between earth (beneath your feet) and the earth on the power supply (lets say the consumer unit) as they are pretty much equal anyway.
As the RCD sees it, the potential difference when the live travels to either of these earths is enough to cause an imbalance between the live and neutral to cause the RCD to trip.

"Mother earth" isn't a good conductor all depends upon how well an object is grounded. That conservatory was probably build on a dry concrete block/wall that had high enough resistance to prevent it from become a true earth. That doesn't mean to say that joe bloggs holding a live wire would be protected from being electrocuted because they aren't making a true connection to earth. Would you stand on the ground you are on now if the supply had no earth cable and feel safe that you wouldn't get electrocuted if you touched the live wire? - I certainly wouldn't want to try!

[beermonsta]

,touching it resulted in a shock.

come to think of it if this is the case then the green house must have been isolated (perhaps by dry wall/rubber seals etc) from ground earth but if the same appiled to the person touching it which "resulted in a shock" - well the only way they would get a shock is if they where earthed (to ground).

How do you think birds sit on 10,000+v (god knows how many amps of current) wires that supply us with power without being electrocuted? - oh yeah - it's because they are not connected to earth!

[Aleman]

If the brewer is isolated enough from earth (e.g. insulating soles, on a wooden step ladder etc) and manages somehow to bridge live and neutral then 30mA may not escape to earth and the RCD will not trip. RCDs are not perfect for that reason, and the fuse or MCB is also extremely unlikely to go, because the resistance of the human body won't easily permit enough current to flow to activate over-current devices.

Been there, done that . . .had that experience . . .4 feet from the fuse box with the RCD device in it. At least 240V is still technically low voltage, and I wasn't doing it with the 600V supplies I used to work with . . . . learning to work with one hand behind your back does have it's uses

[themadhippy]

thinks were arguing at crossed purposes here,several people have stated that an earth is not required for an rcd to operate,my argument is without connecting the electrical supply network to mother earth an rcd will not work were the live is in contact either directly or via a body with mother earth .

[beermonsta]

thinks were arguing at crossed purposes here,several people have stated that an earth is not required for an rcd to operate,my argument is without connecting the electrical supply network to mother earth an rcd will not work were the live is in contact either directly or via a body with mother earth .

i think you are not explaining your thoughts clearly enough. As far as I read no one stated that "an earth is not required for an rcd to operate". And as for "without connecting the electrical supply network to mother earth an rcd will not work" that's the first time you have mentioned such a notion. Most people on the forum, including myself with my limited electrical knowledge assume that the the electrical supply (i.e who ever is supplying the electricity) is connected to "mother earth" otherwise where else would they be connecting it to to get an "earth"???
If your basis for argument is that the supply company don't earth their supply to the same earth that I am standing upon then God help me every time I touch an electrical appliance.

[Kev888]

Ah I see - we were talking before about long protective conductors in extension cables somehow stopping the RCD working, hadn't realised we were now talking about earthing/grounding for the whole supply.

If the supply truly floats (no earth connection of any note) then in all likelihood the RCD will not operate - theres no reason why it should because a person could hold the live (DO NOT DO THIS ANYONE!!!) and in all probability no current of any note would flow to earth. I've never come across this except in purposely isolated supplies, neighbours and substations etc are usually earthed, but its at least a theoretical possibility.

The main point really though is that the two are directly linked - if theres a loss to earth of 30mA+ from a circuit with an RCD downstream of where the loss occurs then the RCD will trip, irrespective of how that 30mA makes its way out. Therefore its better to have it at the plug end of an extension lead rather than the socket if you want to protect the lead as well - even if the extension lead had 'no' earth of its own (but don't try that either!) let alone a long one.

BUT you can still get a shock at less than 30mA, and no shock is safe, and you can still fry between live and neutral if you somehow isolate yourself or the supply from earth so whilst RCDs are very advisable bunging one on isn't a complete catch all.

Cheers
Kev

[Kev888]

Been there, done that . . .had that experience . . .4 feet from the fuse box with the RCD device in it. At least 240V is still technically low voltage, and I wasn't doing it with the 600V supplies I used to work with . . . . learning to work with one hand behind your back does have it's uses

The sound of Kev sucking teeth and wincing.. Its ridiculously easy to do though. I once stupidly touched the paint on a big resistor - not metalic so presumably not setting off any 'conductive' alarms in my brain, but it was on the 1kV line in a strobe light... It was decades ago and I still feel shaky thinking about the result, typically I was sitting in a bucket seat at the time too, so really struggled to get away..

But just to get back to the OP. Yes you can use an extension lead, at least run it from another plug and ideally from another circuit if you can, like the cooker circuit if there is one, but as mentioned make sure its uncoiled fully and if you use an RCD I'd put it at the start of the extension lead to include that in the protection. However make sure its a quality extension lead thats capable of the full 13Amps (not just with '13A' plug on) and try hard not to lay it somewhere where pulling it or tripping on it is likely - but don't get it much longer than you need, as you'll lose power along its length. Monitor the plug/socket too - if they get hot rather than (very) gently warm something isn't working properly.

Upgrading the socket circuit would be a better option of course but usually its more than just using a bigger fuse/MCB, typically the cabling will match the fuse rating and so unless its been over specified that will need replacing with something heavier duty, or having a second one put in. Could be well worth it if you're going to use it a lot though, and the sparky could check that the underlying circuits are up to it too. A properly specified mains circuit with high quality sockets like MK or Crabtree ones would be in a better class if you can justify it.

Cheers
kev

[Dr. Dextrin]

Thanks for thrashing that one out guys - I was happy to sit back and watch as I felt I was losing the will to live...

So we're all safe using RCDs with extension leads again. Hurrah! Unless the supply company suddenly decides to isolate the entire generation and distribution system from earth (must check my T&Cs). But then we won't get electrocuted anyway unless we suck on live and neutral both at once.

I'm off on my hols to Spain tomorrow. That's always good for seeing some quality examples of electrical wiring done well.

[chivelegs]

Bingo! There's the facts I needed!

So I don't need to stretch the mortgage to include a Chinook then?

Upgrading the socket circuit would be a better option of course but usually its more than just using a bigger fuse/MCB, typically the cabling will match the fuse rating and so unless its been over specified that will need replacing with something heavier duty, or having a second one put in. Could be well worth it if you're going to use it a lot though, and the sparky could check that the underlying circuits are up to it too. A properly specified mains circuit with high quality sockets like MK or Crabtree ones would be in a better class if you can justify it.

Cheers
kev