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Labelling a product as ‘Intrinsically Safe’ sounds quite
reassuring. It suggests that the item in question probably doesn’t have any
sharp edges, is
That’s one of the key features of the Edgcumb Metrohm 981. Although it looks a
lot like a regular multimeter, it is not, and in the trade it is more widely referred to as a ‘megger’*. This one has been around
for a while, since the mid to late sixties, and was designed to do just one
thing and that’s measure electrical resistance. (It measures voltages as well
but that’s very much a secondary feature). More specifically it is for testing
the electrical insulation in cables and inside devices, which means it has to
be able to read high values of resistance from thousands to millions of ohms.
If you’re familiar with modern multimeters, that may not sound
like an especially challenging task. Even the cheapest models can measure
resistance, but in general they do so by sending just a few volts through a
circuit. That’s fine for checking cables and devices that operate at low
voltages but when it comes to checking insulation in mains-powered and high
voltage circuits you need something with a bit more oomph. That is why the
Metrohm 981 can do resistance checks by passing up to 500 volts through cables
or circuits. If an insulator has failed or is in the process of breaking down
it might not show up on a low voltage resistance check, whereas the 500 or more
volts from a typical megger is more likely to expose potential faults.
The use of this Metrohm 981 in mines is in no doubt as the meter
is clearly labelled ‘Property of N.C.B.’ That’s the National Coal Board, the
organisation responsible for running coalmines in the UK since 1947. It was
privatised and renamed the British Coal Corporation in 1987. The design has
changed little since it first appeared and there are only two controls. The
rotary switch is for selecting the function: resistance (mega ohms, kilo ohms and
ohms) AC and DC voltage (3 – 9 volts) and Battery Check. Above that is a
spring-loaded rocker switch for selecting the resistance range and battery
check modes. The reason for this apparently unnecessary additional switch is a
safety feature. It's a bit like the 'dead man's handle' in locomotive cabs. It ensures that readings can only be taken when the appropriate
rocker button is pressed, so if the equipment was accidentally switched back
on, with the meter leads still attached, it wouldn’t burst into flames and produce lots of
very unhelpful sparks. On the top of the case there are two pairs of red and
black sockets for the test leads. The ones on the left are for ohms and kilo
ohms resistance and voltage measurement and the ones on the right are for the
500-volt mega ohm test range. The meter has three resistance scales and a small
colour-coded band for battery check and indicating 3 – 9 volts.
The sturdy plastic case is the first line of defence against
sparks escaping the instrument, with strong internal seals surrounding the
battery compartment, meter movement and test cable sockets. It also comes with
a hard carry-case that protects the meter from the inevitable bashing it will
get in coalmines. The 500-volt supply for the mega ohm test is generated by a
simple one transistor blocking oscillator circuit, which makes a high-pitched whine that also helps to
confirm it is working. 500 volts sounds like a lot, and it is enough to give
you a bit of a nip if you’re careless with the test leads, but the current is
very low, in the order of a few milliamps and not capable of making sparks,
even if you touch the leads together as doing so instantly shuts down the
inverter.
How this one came to be at a Surrey car boot sale in 2018, or
thereabouts, is a bit of a mystery. Normally I don’t give test meters more than
a second glance, unless they are clearly old or very specialised, but the NCB
label on this one caught my eye. The seller had no idea where it came from,
what it was for or if it worked and happily accepted my generous offer of 50
pence for it. From the outside it looked like it had been well used but the
hard carry case had kept it safe and the overall condition was quite good. The
meter’s needle moved freely, there were no signs of corrosion so the fifty pee
I paid wasn’t much of a gamble. Worst case, it might yield a few vintage
spare parts. It turned out to be in working order and, with a bit of tweaking
of the calibration pots inside the case, reasonably accurate as well. It only
needed a clean up to make it presentable and on a few occasions has turned out
to be quite useful. Given that the design has changed little over the years it
is difficult to be precise about the manufacturing date but the NCB marking and
components used strongly suggest this one hails from the early seventies.