On Wednesday, 11 December 2019 19:54:02 UTC, Mike wrote:
I have inherited the [unpaid] care of some amateur electric pottery
kilns. The house has wire fuses and no RCD. The supply appears to be TN-
S. There are Henley blocks between the meter and the consumer units.
There is a MET near the main incomer.
One way of a Wylex plastic fuse box has a 40A plug in MCB. This feeds a
10mm csa T&E which is connected to a big metal junction box on the end
of a plastic underground conduit about 45mm dia and about 27 metres
long. I believe it was installed over 40 years ago. The conduit contains
11 individual conductors which appear to be PVC sheathed, not rubber.
They are about 6mm csa, so I guess they are 7/.044; 5 red, 5 black, 1
green and yellow. Two red and two black have been paralleled to feed the
kilns. Is this permitted? Others are not used.
The conduit runs into another big metal junction box in a concrete
sectional garage with the kilns in. Here 10mm csa T&E feeds a small
consumer unit with an 80A RCD, and a 40A MCB which feeds a Henley block.
It has a 6mm csa connection from the earth bar to an earth rod. Should
this be removed?

Each of the 4 kilns is hard wired into the Henley block with modern T&E,
some 10mm csa and smaller kilns with 6mm csa. Each kiln has a small
Wylex consumer unit nearby with an isolator and a MCB, either 40A or 32A
to suit. Each kiln has a 'door open' interlock switch which prevents the
contactor from energising. Each has a programmer which controls the
temperature rate rise by turning on and off the whole kiln, which it
monitors with a thermocouple pyrometer.
The highest load kiln draws 38A. When the mains was 237v there was a
voltage drop of 7 volts when this turned on.
I am not going to modernise the house wiring, [not my job squire] but
I'd like to bring the kiln room up to something like spec. I'll start by
printing laminated instructions about isolating the kilns before loading
them with pots, and having only one firing at a time.
There are 2 questions hidden in this long message, and I'm seeking
advice.

How many kilns are there? What's the current draw of each? AIUI a 40A way is at the limit of an old Wylex's ability, so if just one kiln eats 38A and you have several.... erm, well, that could be a problem. Lack of RCD is unlikely to meet current standards.

A 40yr old interlock system also might not meet modern requirements, but usually old wiring doesn't need to. But several kilns suggests it probably does. More info needed really.


NT