Failed RCD socket given post-mortem out of curiosity

[Midwest]

Amazing. I thought there was a little man inside, stood on a bucket, which he jumped off when the electric got too much.

 

[DPG]

Amazing. I thought there was a little man inside, stood on a bucket, which he jumped off when the electric got too much.

Man/woman/gender neutral person surely.
(or should that be gender live & neutral?)

 

[Midwest]

Man/woman/gender neutral person surely.
(or should that be gender live & neutral?)

Only a bloke would be daft enough to stand there for 32 years.

 

[Lucien Nunes]

When I first saw the board and before I read your comment, I took C1 to be the dropper, but a quick look at the tracks revealed that to be entirely R2, totalling 40k on the DC side of the bridge. So there's around 5mA available to power the amplifier at whatever voltage that zener is. One the one hand, a resistive dropper is wasteful. A 0.1μF cap could have saved something approaching a watt of dissipation and relative to the overall cost it's not a huge extra. But then, unless it was vastly overrated for voltage making it bigger and more expensive still, it would have suffered the same fate as most X-cap droppers, losing capacitance through repeated self-heal events and stopped the unit working after much less than 30 years.

The actuator coil seems to be powered separately by half-wave rectified AC (across one diode of the bridge) via that 47k (R3?), and flywheeled by D9. That scheme makes sense because it utilises the coil's inductance to reduce the resistor loss for a given average coil current, compared to feeding it from the bridge DC output. I can't see what actually trips it - is it shorted by the op-amp's output?

 

[HappyHippyDad]

When I first saw the board and before I read your comment, I took C1 to be the dropper, but a quick look at the tracks revealed that to be entirely R2, totalling 40k on the DC side of the bridge. So there's around 5mA available to power the amplifier at whatever voltage that zener is. One the one hand, a resistive dropper is wasteful. A 0.1μF cap could have saved something approaching a watt of dissipation and relative to the overall cost it's not a huge extra. But then, unless it was vastly overrated for voltage making it bigger and more expensive still, it would have suffered the same fate as most X-cap droppers, losing capacitance through repeated self-heal events and stopped the unit working after much less than 30 years.

The actuator coil seems to be powered separately by half-wave rectified AC (across one diode of the bridge) via that 47k (R3?), and flywheeled by D9. That scheme makes sense because it utilises the coil's inductance to reduce the resistor loss for a given average coil current, compared to feeding it from the bridge DC output. I can't see what actually trips it - is it shorted by the op-amp's output?

Oh Lucien. It was fully explained in post #11 you silly bean.

 

[pc1966]

When I first saw the board and before I read your comment, I took C1 to be the dropper, but a quick look at the tracks revealed that to be entirely R2, totalling 40k on the DC side of the bridge. So there's around 5mA available to power the amplifier at whatever voltage that zener is. One the one hand, a resistive dropper is wasteful. A 0.1μF cap could have saved something approaching a watt of dissipation and relative to the overall cost it's not a huge extra. But then, unless it was vastly overrated for voltage making it bigger and more expensive still, it would have suffered the same fate as most X-cap droppers, losing capacitance through repeated self-heal events and stopped the unit working after much less than 30 years.

I was a bit surprised that it was mostly a resistive dropper, also a cap allows DC IR testing (in theory, depending on how the coil trips, etc)

The actuator coil seems to be powered separately by half-wave rectified AC (across one diode of the bridge) via that 47k (R3?), and flywheeled by D9. That scheme makes sense because it utilises the coil's inductance to reduce the resistor loss for a given average coil current, compared to feeding it from the bridge DC output. I can't see what actually trips it - is it shorted by the op-amp's output?

There seems to be CSR1 and CSR2 to the left of the coil and I think they are thyristors but not checked, I guess it has some ~340V charge on one of the other caps that is dumped in to the coil to trip the breaker. The thin wire and size makes it look unlikely it would go with the 10-20V of an op-amp output.

I kept the PCB from the bin, might try and get better photos.

 

[Lucien Nunes]

I'm glad you pointed out the thyristors - I was expectng there to be one or two but I hadn't spotted them over in that corner, even though the silkscreen clearly shows CSR2*. It's hard to follow their connections behind the flying leads though.

The coil circuit I was looking at starts at the positive side of the bridge, the track carries on past the dropper and round the very edge of the board outside the mounting screw, picking up D9 cathode and ending at one coil pin. The other side of the coil launches two tracks, one of which hads back to D9 anode, connecting also to the test button contact post and one end of the 47k resistor that lies alongside the coil (not 100% sure of its number). That goes straight to the AC side of the bridge meeting the black main power flylead. So I get the impression that absent the thyristor, the coil would remain permanently energised, but it is shunted by firing the thyristor (which is OK because the 47k limits the current). Can you see if this is correct or whether I am misreading the track routing.

I have sometimes wondered why SRCDs to BS 7288 have developed along significantly different lines to BS 4293 and EN 61008 RCDs. I've never read BS 7288 though, so perhaps there is some specific provision that resulted in the different mechanisms used. Or is is just the layout constraints and actuator design that made the conventional toggle mechanism unsuitable for adaptation into a socket-outlet or FCU?

*Why is it that we speak (or used to speak) of SCRs, but the most common component designator was CSR?

 

[Lucien Nunes]

Oh Lucien. It was fully explained in post #11 you silly bean.

Amazing. I thought there was a little man inside, stood on a bucket, which he jumped off when the electric got too much.

Remember I am the king of metaphors. Just for @HappyHippyDad I will do one explaining an RCD in terms of train loads. I think our weighbridge operator might have something to say about differential current:

Easiest explanation for PF

What's the easiest explanation for power factor that anyone's come across? To people who understand Volts, Amps and KW's. I often get asked and realise I confuse people by over-complicated answers!!

www.electriciansforums.net

 

[pc1966]

I'm glad you pointed out the thyristors - I was expecting there to be one or two but I hadn't spotted them over in that corner, even though the silkscreen clearly shows CSR2*. It's hard to follow their connections behind the flying leads though.

It looks like one is used for the RCD trip but the other is li ked to earth so (I presume) if neutral is at an elevated voltage the socket is disconnected as well.

*Why is it that we speak (or used to speak) of SCRs, but the most common component designator was CSR?

No idea, but unexpected ordering of letters often suggests a French influence. I also remember that semiconductor diodes were often given the part number CR1, etc, as short for "crystal rectifier" so possibly some connection there?

Thytristor is very much a British English name, analogous to the Thyratron valve, and shows our historic love of Greek words.

 

[Lucien Nunes]

Going a bit OT but yes 'thyratron' is a descriptive name, 'electron gate'. Thyristor is a bit more tenuous because it borrows '-istor' from transistor to imply a solid-state equivalent to the thyratron, which it is, but it lacks the linear 'transfer-resistor' characteristic that led to the name transistor. Although, the transistor itself doesn't behave like a resistor anyway, more like a current source. On other component designators, I suppose semiconductors generally have had some deviant letters - CR for diode, Q or VT for transistor, U for integrated circuit - so CSR doesn't break the mould.

Going back to the SRCD, did you find what actually killed it? Having got it apart on the bench, I wouldn't be able to resist repairing it, even if I didn't intend to use it. Assuming it's not something like the coil failed which would be unlikely.