V.O.E.L.C.B.

I just happen to have a VOELCB in my garage, I really must get rid of some of the stuff in there!


250 mA passing through the actual device from terminal F to E?

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Knock me down with a feather!


I never thought to turn the cover over. 

Whilst I was measuring the resistance of the VOELCB coil I didn’t take into account it will be a resistance in series with the earth electrode. ?

I wouldn't bet on a modern RCD being more reliable than a 1970s Crabtree voelcb, in the sense of its chance of failing to operate in the next x years.  The old ones were very solidly made - I wonder how much more they cost than a £25 80 A 30 mA RCD. But as the voelcb relies on the earthing wire (black cloth over small-gauge tinned copper?) and electrode (pipe with mushroomed head), the whole system reliability might well be worse, quite apart from its lack of protection for paths that don't go through that electrode. 


I took down a classic Crabtree last year when "clearing up" the electrics at the house where I grew up.  It was an amusing arrangement that I'd quite forgotten, made when - as an impecunious early/mid teen - I'd spent all my saved money on a 50 m drum of  2-core 4 mm2 SWA cable to go down to my shed/workshop. (What a lot of load that cable suffered later: for one thing, arc-welding from the mains using a watering can of brine as a 'ballast'.) Clearly I wanted some degree of spade protection for the cable, but I didn't want any fault in the sheds to trip an upstream RCD, and also didn't want to save money again for a time-delayed upstream RCD. The Crabtree thing, already old and not within regs, was available anyway. So the solution was to connect the SWA to the 'F' (frame) terminal, and connect the 'E' (earth) terminal to the neutral coming out of the switch contacts - doubtless after checking that it was indeed make-first and break-last.  No need to analyze whether this should have been awarded a C3 or C1 or whatever .. it's gone now, and building regulations weren't the same in those days, and age does bring added caution. The cable was disconnected years ago when the sheds started crumbling, but the breaker still tripped happily on the test button last year, after decades of normal use and further decades of unconventional use without any press-to-test being done. Earlier this year I was in a lab with many benches, each with several RCDs (for normal sockets, a 230/400V source and a 130/230V source), hearing an introduction about the RCDs being there for safety: I idly pushed a test button, with no result.  It turned out it wasn't just the button, and that the RCD was by no means alone among its companions. They had the 70 or so RCDs replaced, although they all looked fairly new to me, and had a respectable name. Perhaps the test buttons will be used more with the new ones.


Unfortunately I don't have the 1930s or 1960s 842 standards.  Even if standards define the devices by maximum operating voltage (together with maximum electrode resistance), a particular device does have a tripping current that could be determined by its operating voltage, tripping-coil impedance and the maximum permitted electrode resistance.  A strong proponent of (VO)ELCBs versus the competing 'PME' in the late 1930s and early 1940s wrote this (Gilbert, 1941): the article and comments on it suggest that there already had been devices in use with 15 mA trip, and 30 mA was quite common, but something nearer to 60 or 80 mA was advised in order to allow a more robust coil (there was worry about damage from e.g. nearby lightning, though one comment mentions no recorded trouble). I wonder what the nice 1940s book was [in the earlier postings]?


Colour codes:  the teutonic influence spread north too ... here's an example of old meets new in Sweden. 

At first I didn't think I'd ever get used to red wires going to the exposed parts.


[Added:  nice pictures that went up while I was writing.  Mine was just like that one. Trip coils would typically have significant reactance as well as resistance. And there wasn't a worry about DC components in leakage currents with voelcbs.]

I can make a RCD trip using the continuity tester as it’s pushing out 200 mA, I cannot make the VOELCB trip with it.

Don't trust the tester to push out 200 mA.  Measure it with another meter in series [as well as the VOELCB coil].  Or connect the coil to a car battery.   The tester should test with e.g. 200 mA to a near-zero resistance, such as a conductor through an RCD.  But if the VOELCB's coil really is tens or more of ohms of resistance, your tester is unlikely to produce the required voltage to get 200 mA [or 35 mA] - it probably only runs off a few AA batteries.


(You see ... I really don't want to give up my faith in the reliability of the old stuff, until forced.)