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Widget Of The Week

Metrohm 981 Intrinsically Safe Test Meter, 1970?

Labelling a product as ‘Intrinsically Safe’ sounds quite reassuring. It suggests that the item in question probably doesn’t have any sharp edges, is

unlikely to give you a shock or be a choking hazard for infants. In the highly dangerous business of mining, and allied trades (petrochemical refining, processing, and so on), it has a very different and much more precise meaning. Basically it’s a guarantee that a piece of equipment is incapable of producing enough heat or electrical energy to ignite volatile chemicals, vapours and gasses. In other words, the last thing you need when servicing or fault-finding electrical stuff in a coal mine filled with explosive gasses, is a test instrument that produces sparks… 

 

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.

 

What Happened To It?

*Megger is actually the name of one of the companies that developed the earliest insulation testers; it was registered back in 1903, and they are still in business. Meggers, in the wider, generic sense, continue to be a must-have instrument for electrical engineers and the modern and outwardly unchanged descendents of the Metrohm 981 are still with us. Edgcumbe, the company responsible for the original design was founded in 1900 and following a series of takeovers and buyouts Metrohm branded test instruments are still being made.

 

Although most UK coalmines were closed in the 1980s the need for test meters like this one in other areas of industry has never gone away. Until recently NCB labelled Metrohm meters rarely found their way on to the second hand market but within the last year or so there’s been a steady stream of them appearing on ebay, suggesting someone somewhere has been having a clearout. Usually this results in fairly settled pricing, but a quick trawl through the online listings shows prices, for this particular NCB model, ranging from a reasonable £10 to a ridiculous £150. Vintage meggers from the 1960s onwards, even ones with Coal Board associations, are not especially rare or exciting. Even if you want one to use you won’t have to spend a fortune -- £20 to £50, say -- and the 981 is still a very useable instrument.

There is, however a collector’s market for the really old ones. It is a very different ball game and not unknown for wooden-cased examples from the late nineteenth and early twentieth centuries, especially if they have shiny brass fittings and a winding handle, to change hands for hundreds of pounds.


DUSTY DATA

First Seen:                   1970?

Original Price:              £?

Value Today:                £10.00 (1120)

Features:                      Electrical insulation & voltage tester, kilo & mega ohm resistance ranges, low and high voltage (500 volts) test modes, AC/DC low voltage check, battery check, intrinsically safe operation, hard carry-case

Power req.                     1 x 9-volt PP3 type battery

Dimensions:                   130 x 84 x 60mm

Weight:                          500g

Made (assembled) in:     Edgcumb, Glasgow

Hen's Teeth (10 rarest)    6



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