dodgy RCD

[Rob_the_rich]

@Sidewinder was it you who was asking for dodgy RCDs?

Recently tested an MK Sentry DP two module 80A RCD (tested from plug socket) and it failed to trip on all x tests. I then pressed the test button and it tripped. But still failed to trip with the auto test. TNS supply, Good Zs results.

MFT is recently calibrated and tests worked fine on my extension lead RCD.

The next day I went to site with replacement but thought to test again before taking it out and it passed all tests.

Swapped it out anyway and the old one is sitting in the van.

Did you want it?

Edit; test button has probably never been used for about 8 years since the CU was installed.

 

[Evans Electric]

Thats interesting Rob ,    note that the advice on RCD testing is to leave the test button until last because for reasons I,ve never found out , the device may do what yours was doing but the test button seems to clear that problem .  

But if your tester didn't trip it , neither would a fault.

How important is the quarterly test on the button we ask ourselves. ?     Does that "clear"  the coils or does mechanism get sticky?  

Lets face it , there won't be many customers hitting the test buttons every quarter , not gonna happen , I've NEVER met anyone who did it TBH.  and on an RCBO board  they all need testing . 

I've had test buttons that didn't work but tripped with the tester . 

I've fitted brand new ones that didn't trip at all.  

I've  fitted new boards , gone back 6 months later to add a socket and found the RCD I fitted no longer trips.   

 

[apprentice87]

That is why a TT installation is not a wise move.. Better off with TNCS, ["exported" or otherwise] than TT......

An RCD is the worst most unreliable contraption there is. Do not get me wrong, i would rather have one everytime, but as a sole means of achieving ADS under earth fault conditions they are dire..

john..

 

[steptoe]

and dropping an incoming neutral on a TNCS system is satisfactorily safe?

 

[apprentice87]

No. very very far from it, but a lot less likely to occur than a faulty rcd... Unless of course you all know different. How often do you come across TNCS neutral problems in the real world?? I would not be at all suprised if it was getting a more regular thing...

john..

 

[Andy™]

a proper TNS would be better...

 

[steptoe]

No. very very far from it, but a lot less likely to occur than a faulty rcd... Unless of course you all know different. How often do you come across TNCS neutral problems in the real world?? I would not be at all suprised if it was getting a more regular thing...

john..

not that many actually,

maybe 10 or 12,

a proper TNS would be better...

or a proper PME 

 

[apprentice87]

not that many actually,

maybe 10 or 12

OMG, that is horrific!!!... If any of us owned the cables the HSE would slap a prohibition order on us, but the DNO's, above the law..

john..

a proper TNS would be better...

It most certainly would.......

john..

 

[Adrian]

I've came across 1lost neutral in 30 years of trading and lost count of faulty rcds

 

[apprentice87]

I've came across 1lost neutral in 30 years of trading and lost count of faulty rcds

To be honest that is what i would have expected..

john..

 

[kerching]

In over 40 years

loads of faulty RCDs....but NONE in the last few years

3 lost neutrals...one was found on a JCB bucket

 

[Sidewinder]

@Rob_the_rich it was myself who asked about the incidences of faulty RCD's it is more the numbers of occurrences rather than getting hold of the devices to be honest.

As far as failed Neutrals to failed RCD's in my experience it's about 10 RCD's to 1 failed N.

 

[Evans Electric]

Never came across a lost neutral in ## years s  but my mate has .  A row of new houses with 400 volts running through them and the tarmac drives melting above the supply  cable trench  apparently  ....appliances blown to bits and current flowing down the  , then,  6mm bonding.