11-10-2012, 09:37 AM
Hi,
My above reply was dashed off in a bit of a rush, so perhaps I ought to try and clarify things a bit...
First, I agree that if you are exporting the earth, then yes, do use the 3rd core. I did write that in my first reply, but took it out after deciding that exporting the earth was A Bad Thing.
Regarding the issue of exporting the earth, I'll explain my background with this:
I'm obviously not an electrician, but I have some experience in these matters because we have run courses for OB engineers, who have to plug their vehicles into a range of different types of supplies. From first principles, we've had to explain the different types, and what the risks are with each. And of all of those, PME supplies are considered to be the worst.
The risk from PME supplies concerns an open-circuit on the Neutral, ahead of the RCD on your incoming mains. What happens is that everything in your house, including earth, goes high. The RCD doesn't trip out because it's lost power. So, you have a house that seems dead, yet by simply touching the metal case of any electrical device, you could get a nasty shock.
This page explains this, with a diagram: http://www.tlc-direct.co.uk/Book/5.6.3.htm
Interestingly, BBC OB vehicles are regarded as caravans in the eyes of the law!
Caravans, hence BBC OB vehicles, can not be connected to PME supplies. In our case, quite how the rigger establishes what type of supply the venue has is another matter
This is a risk with your house as it currently stands, but of course what you're doing is new work, and has to comply with the standards. If you export your earth, the new work also has this risk of earths becoming live in the event of a neutral O/C ahead of your incoming RCD. It's almost certainly not allowed.
So, by providing an RCD feed in the house, with a local earth spike at the caravan, and not exporting your earth, you have converted the supply from the house to a TT supply. Should the incoming neutral to your house fail O/C, then at least the exposed metalwork of any Class 1 appliances in the caravan will be at ground potential.
Of course, your earth spike results in a fairly high earth-loop impedance, so a live-to-earth fault might not pass enough current to blow the fuses, which is why an RCD on the feed is essential.
Does that help to make things clearer? Now that I've slept on it, and double-checked a couple of things here at work, I'm much happier that I understand this and can try to explain it better if needed.
Cheers,
Mark
My above reply was dashed off in a bit of a rush, so perhaps I ought to try and clarify things a bit...
First, I agree that if you are exporting the earth, then yes, do use the 3rd core. I did write that in my first reply, but took it out after deciding that exporting the earth was A Bad Thing.
Regarding the issue of exporting the earth, I'll explain my background with this:
I'm obviously not an electrician, but I have some experience in these matters because we have run courses for OB engineers, who have to plug their vehicles into a range of different types of supplies. From first principles, we've had to explain the different types, and what the risks are with each. And of all of those, PME supplies are considered to be the worst.
The risk from PME supplies concerns an open-circuit on the Neutral, ahead of the RCD on your incoming mains. What happens is that everything in your house, including earth, goes high. The RCD doesn't trip out because it's lost power. So, you have a house that seems dead, yet by simply touching the metal case of any electrical device, you could get a nasty shock.
This page explains this, with a diagram: http://www.tlc-direct.co.uk/Book/5.6.3.htm
Interestingly, BBC OB vehicles are regarded as caravans in the eyes of the law!
Caravans, hence BBC OB vehicles, can not be connected to PME supplies. In our case, quite how the rigger establishes what type of supply the venue has is another matter
This is a risk with your house as it currently stands, but of course what you're doing is new work, and has to comply with the standards. If you export your earth, the new work also has this risk of earths becoming live in the event of a neutral O/C ahead of your incoming RCD. It's almost certainly not allowed.
So, by providing an RCD feed in the house, with a local earth spike at the caravan, and not exporting your earth, you have converted the supply from the house to a TT supply. Should the incoming neutral to your house fail O/C, then at least the exposed metalwork of any Class 1 appliances in the caravan will be at ground potential.
Of course, your earth spike results in a fairly high earth-loop impedance, so a live-to-earth fault might not pass enough current to blow the fuses, which is why an RCD on the feed is essential.
Does that help to make things clearer? Now that I've slept on it, and double-checked a couple of things here at work, I'm much happier that I understand this and can try to explain it better if needed.
Cheers,
Mark
