Chalet/Caravan Earthing?

[pwdrive]

Jeffrey, I'll try that, going to pick up a couple of 4 ft x 5/8 ths rods and coupler etc in about an hour or so from City Electrical, they also said they could get a Martindale EZ150 in a couple of days.

I shall report if I make any progress.

Lawrence.

[pwdrive]

Later in the day.....First 1.2 meter rod went down ok, tough in places but not too bad, My old Avo was checked on AC, 230 vac registered 210 vac on the Avo, a bit out, anyways connected the Avo set to 250 vac between the earth rod and the MET, no indication, set to 2.5 vac range, the needle gave about 100 millivolt reading, measured ohms between the earth rod and the MET and it kicked the wrong side of zero so nothing conclusive:

   

Another rod was attached and driven down, approx 2.4 meters now in the ground, checked the ac again, about 75 millivolt this time, meter still deflecting the wrong way when measuring ohms:

   

I need an earth loop impedance test, I asked at the electrical factors about the Martindale EZ150 he reckons it wont check earth loop impedance? I'm not too sure on that, I might get one anyways.

Lawrence.

[Mark Hennessy]

The EZ150 is a nice little unit. We use them a lot at work, and I bought one for home use as well.

Yes, they do measure earth loop impedance. But they only give you an indication of the impedance:

• 0 - 1.7Ω
  
• 1.7 - 5Ω
  
• 5 to 10Ω
  
• 10 to 100Ω
  
• 100 to 200Ω
  
• 200 to 500Ω
  

There are 3 LEDs to indicate these 6 possibilities. The appropriate LED flashes green to indicate the first three, and red to indicate the second - the labelling is pretty clear once you've figured it out. If you look at the picture here, you'll see the middle LED on the top row is red, and that means 100 to 200Ω

This unit won't trip an RCD because it takes narrow pulses of current.

[pwdrive]

I thought they did too, the bloke at City Electrical reckons not and only checks the CPC, maybe he's got is wires crossed, I looked at the on line manual and it mentions earth rods etc, I'll get Mrs Hillbilly to buy one

I reckon there's enough give in the ground to get another length of rod down or at least part of one, I read that deeper is better.

The bloke on the counter tried to flog us some marconite, but realistically for me I could only dig a hole down to about 3 or 4 ft for a narrow hole, that might not be deep enough to get 200 ohm or less with marconite

Lawrence.

[pwdrive]

The plot thickens.....Not long phoned up Martindale UK to ask if the EZ150 would do an earth loop impedance test on a TT system, non of their staffs knew.....I'll have a re think.

Lawrence.

[Mark Hennessy]

Of course it will. But if your ELI is above 500Ω, it will report that your earth is open-circuit.

[pwdrive]

Mrs has ordered the loop tester, meanwhile the TT system project continues at the chalet, the chalet CU's 2 pole isolator switch has been replaced with a 30ma 63A 2 pole RCD, disconnected the exported earth and bonding from the house to the chalet (don't do that folks) connected the earth rod to the earth terminal in the chalet CU, wired up a 7.8k resistor (the only suitable resistor I had) between live and earth in a 13 amp plug, plugged it in and switched on the socket, the RCD tripped ok, the RCD in the house CU that feeds the chalet CU did not trip, I tried the test four times, not conclusive by any means I know, the loop tester when it arrives will give us an idea as to what's what, if the earth loop impedance turns out to be over 200 ohms I'll have to sink some more rods down in hope....

The power to the chalet in the meantime has been disconnected.

Lawrence.

[ppppenguin]

You should really only have 1 RCD in the feed. It's not unsafe to have 2 but if there's a fault you'll never know which one will trip first. The only way to get discrimination with cascaded RCDs is to have one with a time delay in the upstream position.

RCD at house end: Protects the cable but nuisance if it trips

RCD at chalet end: Easy if it trips but cable not protected.

An armoured cable doesn't actually need to be protected by an RCD so I would tend to put one at the chalet. The feed to my garage has RCD at house, but this was because it fed several other outdoor sockets. ALso because the temporary (lasted 10 years) feed to the garage was a bit of heavy flex strung along the fence. That needed RCD protection.

[pwdrive]

The house CU is a split box, each group of circuits is 30 ma RCD protected, the SWA cable to the chalet is taken from one of those groups via a 50 amp MCB, trouble is there's no more spare ways in the CU where I can take a non RCD protected MCB feed from for the chalet, cascading the chalet 30 ma RCD with a time delay RCD would be a safety compromise so far as I know.

Lawrence.

[ppppenguin]

The standard way out of this problem is a Henley block to split the tails ahead of the CU. Then a switch for the feed to the chalet. Provided there is overcurrrent protection for all circuits in the chalet and the total of the MCBs in the chalet don't exceed the SWA rating then you may not need a fuse or MCB at the house end. Probably best to use a 50A fuse/MCB.

As I said before, cascaded RCDs are safe, if possibly a nuisance. Don't bother with time delay RCDs, they are really only for more specialised applications. Such as on a farm where you might use a 300mA delay RCD as the main switch.