RCD blowing cutout fuse

[JohnW2]

Hi folks,

I've just had a 'panic call' from one of my daughters. Her house has an ~12-year-old electrical installation (PME) with a single-RCD CU. Being well-educated (by me!), she has just tested the RCD by pressing its test button. There was apparently a very loud bang when she pressed the button and the meter now seems to be without power (no indicator lights and digital display gone blank).

I can but presume that some malfunction in the RCD caused a L/N fault which caused the cutout fuse to operate when the RCD was forced tooperate. It's a new one on me - have others experienced this?

Kind Regards, John

 

[Spark123]

Have had it happen before (albeit was an upstream 63A MCB that operated).
In my case the RCD failed to operate when the test button was pressed and the test resistor cooked to the point that it shorted out and went with a BANG!!
Don't know why but somehow the test button contacts managed to weld themselves together - I certainly didn't hold the button in for more than a short period but was a few seconds after when it went bang.

 

[matt1e]

Yes had it too, same as spark123's situation it took out a fuse in an upstream section board

Matt

 

[JohnW2]

Have had it happen before (albeit was an upstream 63A MCB that operated). In my case the RCD failed to operate when the test button was pressed and the test resistor cooked to the point that it shorted out and went with a BANG!!

Interesting. From what I understand, her RCD did operate, but I could be wrong.

Don't know why but somehow the test button contacts managed to weld themselves together - I certainly didn't hold the button in for more than a short period but was a few seconds after when it went bang.

Hmmm. The resistor would obviously have to go S/C before there would be any risk of the test button contacts welding, but once the resistor was S/C, it would presumably only take a very brief touch of the button to cause a big bang! It's hard to see what would cause the resistor to 'spontaneously' go S/C when it's not normally carrying any current, or close to any heat generation.

I'm obviously going to have to go and determine exactly what has happened. Watch this space!

Kind Regards, John

 

[JohnW2]

Have had it happen before (albeit was an upstream 63A MCB that operated). In my case the RCD failed to operate when the test button was pressed and the test resistor cooked to the point that it shorted out and went with a BANG!!

Interesting. From what I understand, her RCD did operate, but I could be wrong.

I've obtained confirmation that the RCD did operate (and will still reset). I'll soon be off to investigate (fortunately not too far away), and will report back.

Kind Regards, John

 

[RF Lighting]

I've had one explode when pressing the test button, but it didn't take out any upstream device.

You will need to replace the RCD first of all, and then get the DNO out to replace the cut out fuse for you.

 

[JohnW2]

I've had one explode when pressing the test button, but it didn't take out any upstream device.

Interesting - that's at least three of you, then. I can't really think of anything much other than the test resistor going S/C that could cause this - and, as I said, I find it rather hard to understand why that should happen to the resistor when it spends 99.99% of it's time 'doing nothing'. I suppose another outside possibility would be some sort of 'collapse' of the test switch, such that pressing the button caused conductors within the RCD to come into contact.

You will need to replace the RCD first of all, and then get the DNO out to replace the cut out fuse for you.

Indeed.

Kindest Regards, John

 

[Spark123]

The test resistor is usually only in the circuit for <40mS, the Wylex one I have on the shelf has a 1.8K resistor in it hence around 130mA will flow at 240v.
If I have done my maths right it equates to somewhere around 30Watts, the resistors are certainly not 30W flavour hence if they are placed online constantly they will get toasty in no time!

 

[JohnW2]

The test resistor is usually only in the circuit for <40mS, the Wylex one I have on the shelf has a 1.8K resistor in it hence around 130mA will flow at 240v. If I have done my maths right it equates to somewhere around 30Watts, the resistors are certainly not 30W flavour hence if they are placed online constantly they will get toasty in no time!

Yes, your maths is right - 240V across 1.8K&#937; will result in 32W being dissipated.

However, isn't the arrangement such that current (hence the 32W of heat generation) will only flow until the RCD operates - which should not take much more than 40mS at most with a current of 130mA - probably not long enough to produce toast. If they've wired them such that current flowed through the resistor after the RCD operated, for as long as one kept one's finger on the button, I would have thought that we would be hearing of 'bangs' all the time - there's nothing in the 'test instructions' which warns consumers only to press the button very briefly.

I've got to wait for daughter to get back from Tesco before I can investigate,but hopefully that will be soon!

Kind Regards, John

 

[Iggifer]

I've got to wait for daughter to get back from Tesco before I can investigate,but hopefully that will be soon!

Has she gone to pick up a new RCD and service fuse

 

[RF Lighting]

Don't forget to take your camera John. We love pictures

 

[EFLImpudence]

However, isn't the arrangement such that current (hence the 32W of heat generation) will only flow until the RCD operates - which should not take much more than 40mS at most with a current of 130mA - probably not long enough to produce toast. If they've wired them such that current flowed through the resistor after the RCD operated, for as long as one kept one's finger on the button, I would have thought that we would be hearing of 'bangs' all the time - there's nothing in the 'test instructions' which warns consumers only to press the button very briefly.

I have no experience of this problem but -

looking at the circuit diagrams it would seem that the test button circuit is on the load side of the RCCB so would only be energised until the breaker trips.
Assuming it tripped in the proper time this, presumably, would not cause overheating but if the device were faulty how long did it really take?


Also, I have read many threads about the wiring of RCDs and I think it is accepted that wiring the other way round, i.e. supply to the bottom does not matter but this would then mean the test button circuit would be on the supply side causing the current to flow until the button was released.

Either way, if that were the cause, it does seem to be less than satisfactory.

 

[JohnW2]

Has she gone to pick up a new RCD and service fuse

Nothing would surprise me with my daughter!

Kind Regards, John

 

[Spark123]

The test resistor is usually only in the circuit for <40mS, the Wylex one I have on the shelf has a 1.8K resistor in it hence around 130mA will flow at 240v. If I have done my maths right it equates to somewhere around 30Watts, the resistors are certainly not 30W flavour hence if they are placed online constantly they will get toasty in no time!

Yes, your maths is right - 240V across 1.8K&#937; will result in 32W being dissipated.

However, isn't the arrangement such that current (hence the 32W of heat generation) will only flow until the RCD operates - which should not take much more than 40mS at most with a current of 130mA - probably not long enough to produce toast. If they've wired them such that current flowed through the resistor after the RCD operated, for as long as one kept one's finger on the button, I would have thought that we would be hearing of 'bangs' all the time - there's nothing in the 'test instructions' which warns consumers only to press the button very briefly.

I've got to wait for daughter to get back from Tesco before I can investigate,but hopefully that will be soon!

Kind Regards, John

Yep The test resistor has it's own contact that opens when the unit trips so the only reason it doesn't toast is it is only online for a very brief period, sub 40mS. Go above this short period though, as in when something fails and things can go bang.

The contact in the test circuit is also there for isolation reasons i.e. if the RCD is off then pressing the test button wont introduce voltage to the output side.

We'll look forward to the pics!

 

[bernardgreen]

Don't forget to take your camera John. We love pictures

Especially FLASH photography