If it really is TT then it never complied. A 30 mA RCD up front is the easiest fix. Why Ze is so low is a mystery you are unlikely to solve and there is little point anyway. In Bristol there are quite a few like this, where Earthing is quite marginal, and the age of the head is probably 60s so perhaps no one has done much for a long time. I assume the supply is overhead from the Brown incomer cables?

Indeed David, new overheads with P.M.E. earthing available on request. Fortunately the house owners are having a new E.V. charging point and a new consumer unit installed soon.

Z.

15 ohms is a perfectly credible electrode resistance for paths through the plumbing - 4ft of rod plus a tens of  m of horizontal pipe to the plastic water meter.

However, it should have had a VOELCB when new, and some sort of up-front RCD nowadays - a PSSC of about 15A  may just about blow a lighting fuse, but not a fat lot else.

Mike.

Too dangerous to walk away and leave.

Do a global insulation test on the whole installation and if you get a reasonable test result simply swap the main switch and the 30 mA RCD around in the existing consumer unit, so that the RCD protects the entire installation until the consumer unit is replaced.

With the customer's permission that is a good idea.

Indeed gas pipes or water pipes must not be used as earth electrodes. 

542.2.6.

Z.

I can't imaging in the 80s or 90s this arrangement ever complying. Is the installation relying on the old gas pipe as an earth electrode?

Probably back in the 14th Ed relying on the water (or gas) pipework would have provided an acceptable earth - 15Ω now is probably the result of the mains down the street being converted to plastic.

   - Andy.

But there are 10.00mm2 green and yellow bonding cables there. A newish earth rod,  and evidence of brown and blue rewired cables. Who would leave things like they are?

Z.

Oh, I expect that was a plumber I've seen that many times as they faff with their favourite "Bonding"! There will of course be no documentation.

There is another point Z and that is that bonding to the incoming Iron mains may have given a reasonable figure and the regulations "rely" may well have been missed, and the latest wiring was probably done by someone who didn't do any testing and was blind and stupid, so you get the situation you found. As I said, fairly common around here.

Quote. "And since diesel engines require more powerful engine starters, you can expect to use over 500 amps to jump-start a small diesel engine."

We are only looking at four times the average current needed to jump start a small diesel engine when the battery is flat. And we use pretty poor foreign made clamps for that. So good quality earthing clamps should be able to carry 2,000 Amps for 5 seconds. They may get a bit warm but they will not fail.

Z.