RCBO’s ? ? ? ?

[Richard Burns]

I agree that SP is a bit odd if the device can respond to faults on either live but I think they are only concerned with a fault within an installation. If there is a L to N current difference in that circuit then the line is switched off, since there should then be no current in the circuit (if considering only that circuit) a N-E fault should no longer be a problem.

 

[Knobhead]

I can’t see how you can put a protection devise in that does half a job and say that it’s OK. The whole idea of a RCD is to protect against both L→E N→E faults. Neutral to earth can cause very high fault currents.
Beats me why when you install a CU you do all the testing if you think certain faults don’t matter. We may as well go back to VOELCB’s!

How many 15[SUP]th[/SUP] and 16[SUP]th[/SUP] edition RCD protected boards have been ripped out to be replaced by a bag of s**te. The only improvement is to the installers bank balance!

As far as I’m concerned now, they belong at the bottom of the poo pile until they standardise on PD. My regard of this industry has just about hit rock bottom!

 

[RogertheBodger]

Sparty you may be referring to regulation 537.1.2 with concern for isolation and switching where for a TT system the neutral should have to be disconnected. If your using your RCBO for that then yes it should be double pole, but normally it is considered that your main switch in a CU can adhere to this regulation

There is actually no mention to protection in the regulations concerning double pole protection devices with regards to TT systems. The reasons I prefer as I expect you do is that a double pole device with isolate the neutral and will clear a N-E fault, where a single pole device might not

And usually doesn't, under local Regs, DP protection devices are mandatory (Mostly TT systems) but it feels so much....... what? Cleaner, safer, easier to fault find an installation that has DP protection than an installation where all Neutrals are connected straight through.


A point in favor of DP installations is that when you get the call "My RCD is tripping" on your way to the Friday night Lash-up, it's possible to pop in and turn off the affected circuit and ....'see you tomorrow'.

 

[WayneL]

Neutral to earth can cause very high fault currents.

How?

Neutral to earth faults, and the current that flows through them, are reliant upon the 'load' and the resistance of the 'fault' itself to earth. They are likely to be very small.

Perhaps you could explain why a 'neutral' conductor, in a TN system, requires isolation under single fault conditions.

 

[Knobhead]

Perhaps you could explain why a 'neutral' conductor, in a TN system, requires isolation under single fault conditions.

Being an old fart that doesn’t touch domestic. All earth leakage protection I’ve dealt with for the last 40+ years has disconnected all circuit conductors under fault conditions.
I fail to see how it is deemed acceptable to leave a fault connected to the system. The fact that the neutral is disconnected under fault condition will prevent any neutral earth currents.
It begs the question, why make RCD’s DP? I suppose our distribution system is so reliable that external neutral earth faults never occur.

 

[Engineer54]

Many of the major makes, now produce a SP RCBO that switches the neutral. They are not true DP RCBO's as there is no over current facility on the neutral side, just the DP switching... Same physical size as a standard SP RCBO too...

 

[SPARTYKUS]

Have you seen also the RCBOs that are the same size as an
MCB?

 

[Knobhead]

Many of the major makes, now produce a SP RCBO that switches the neutral. They are not true DP RCBO's as there is no over current facility on the neutral side, just the DP switching... Same physical size as a standard SP RCBO too...

That makes them more palatable.
They’re something I’ve never had need of so I’ve never taken an interest in them.

 

[Engineer54]

Have you seen also the RCBOs that are the same size as an
MCB?

Yes, they have been around sometime now in and around Asia. I think the Aussies have been using them for a while too. Strange that they haven't made much of an impact in the UK/Europe to date!!

 

[Deleted member 9648]

Being an old fart that doesn’t touch domestic. All earth leakage protection I’ve dealt with for the last 40+ years has disconnected all circuit conductors under fault conditions.
I fail to see how it is deemed acceptable to leave a fault connected to the system. The fact that the neutral is disconnected under fault condition will prevent any neutral earth currents.
It begs the question, why make RCD’s DP? I suppose our distribution system is so reliable that external neutral earth faults never occur.

Maybe I'm missing something here....but on a TN system protected by OCPD's only,(perfectly within the 'regs' in many installs)...a N-E fault can sit there indefinately......or at least until the next EICR

 

[Knobhead]

Maybe I'm missing something here....but on a TN system protected by OCPD's only,(perfectly within the 'regs' in many installs)...a N-E fault can sit there indefinately......or at least until the next EICR

Yes a N→E fault can and does sit there for years, but any system I’ve worked on where any form of earth leakage protection is provided four pole devices were used to break the neutral.
I’ve had some interesting little fires where more that one N→E or a high resistance neutral has occurred, but they only happened on the “modern” plants. All the older plants didn’t use neutral at all. The star of the transformer was taken down to earth, but that’s as far as it went.
The most fun were the “earth free” systems that had earth leakage fitted. They used DC injection in to the AC and measuring the return DC leakage current through inductors.

As for EICR’s I’ve never see one done anywhere I’ve worked. The time to carry out would be measured in man/years. Major items would have an annual statuary earth continuity (Ductor) test but that was it. They were a farce, due to the number of parallel paths the earth would be 100’s of % better than the line.

The distribution systems weren’t designed, they evolved and grew over the years. It used to amaze me when I’d kill a supply and something totally unrelated switched off. Over the years I’ve blacked out:
The local pub about 5 times.
A health centre.
The local infant school. (the kids thought it was great!)
Half a village.
Street lighting on an “A” road.

Parts of the local 11KV area network was fed from our 33KV intake subs.