Type A RCDs Vs Type AC

[tmus1]

I understand why the increased use of type a rcds are recommended however there’s something I don’t understand. If these type a rcds aren’t tripping at 1x on a normal 50hz ac cycle or at x5 then surely they can’t be fit for purpose because they are not detecting a potentially critical fault. In my understanding an MFT simulates a fault at a specific leakage current in order to trip the rcd. Type ac rcds trip without failure most of the time. But carry out exactly the same test under the same conditions on a type a RCD it fails to interrupt the supply. Obviously there’s a type a test on the mft but why is it acceptable that they fail the ac test when the circumstances and requirement for them to trip under fault conditions on an ac circuit hasn’t changed?

 

[NDG Elecs]

Hager (I think) have released guidance on this stating you should test at 50mA and 250mA for X1 and X5 tests, using the variable setting on your tester.
This is for type A RCDs only IIRC.
It is mentioned in the BBB under 643.8.

EDIT: at 50mA the X1 test no longer applies. Here is the link:

Updated Guidance: RCCB Testing - https://www.hager.co.uk/news-exhibitions-case-studies/18th-edition/updated-guidance-rccb-testing/97806.htm

 

[tmus1]

I have seen that and have taken note however if an RCD even a type a is rated at 30ma should it not operate as if it were also a type ac. If for example joe bloggs plumbing and heating services touched a live cable and it’s only going to trip in o.4 seconds when 250ma is applied it doesn’t sound great. The same RCD then fails to Operate if 150ma or 30ma is applied. Which are potentially lethal levels of leakage current, surely the risk of electrocution is increased. If the rcds fail to trip at 30 or 150ma then surely there’s an issue with the manufacturer not recognising that the devices they are manufacturing are not operating correctly at 30-150ma I’m not yet convinced that Hager has sorted this. writing an article where they have quoted a reg from bbb saying “it’s ok that it doesn’t trip in 0.4 seconds with normal test conditions the bbb says we can make up our own test parameters so just stick a heap more current than what it’s actually rated at through it.”
Don’t know about you but I’m struggling to understand why that’s acceptable you wouldn’t stick lights on a 32A fuse so why test 250ma when logically we should be testing much less to ensure the operation is correct

 

[Wilko]

Hi - I might have missed something - but an RCD with 30mA rating on its faceplate should operate when ramp tested with AC at a value somewhere between 15mA and 30mA. Whatever the RCD type, I'd have thought (?).

 

[tmus1]

Exactly this is the point I’m making why is it that hager are then saying we need to test the type a rcds at 250ma in order to obtain the type of result we want. The amount of leakage current we are then testing with has changed significantly and if it’s not performing below that level then surely it’s not fit for purpose?

 

[Wilko]

Then if an RCD is for Additional Protection it needs to be 30mA or less operating current (Reg 415.1.1) and achieve 40ms or less operate time with 5 times operating current or higher (a note in Reg 643.8).

This does seem a bit odd, as you say. It is a new section in BBB and replaces 612.10 in BYB which made no mention of "or higher". I could perhaps read it as "hit it as hard as you like till it operates within 40ms", which can't have been the BBB intention .

 

[tmus1]

Best set the tester to 500ma test and test the 30 on that, it’ll trip out in no time at all, I’m sure that’s how your supposed to test rcds

 

[Charlie_]

Hager will move the goal posts to suit themselves..
Before metal boards it was stipulated by the manufacturer that RCBOs must not be used on TT systems and now it’s suddenly all ok..
When Hager done their Amd3 roadshow I quizzed them on quite a few things but he couldn’t break out of script, kept repeating the same nonsense.
Even the regs are guilty of it, would appear they are in cahoots with each other.

 

[Dobes_88]

I've used a number of the Wylex Type A miniature RCBO's, have had no problems whatsoever testing at 30mA & 150mA.

 

[NDG Elecs]

I'm not saying I agree or disagree with the guidance, and I was always taught that anything greater than 30mA is not good for the old ticker.
However, Hager are just using what is stated within 61008 according to the link I posted. I agree with Charlies post in the aspect of goal post moving. Don't think it is just one manufacturer though, they would all do it. And yes they are all in cahoots with each other IMO.

 

[Deleted member 26818]

One of the things I don’t understand, is the tripping times of 200ms and 300ms.
Older RCDs BS 7288 and BS 4293 had tripping times of 200ms at 1 x I delta n.
Whereas the three standards listed as being acceptable for additional protection in the 18th edition, BS EN 61008, BS EN 61009 and BS EN 62423, all have tripping times of 300ms @ 1 x Delta n.

Maximum disconnection time for final circuits in TT installations is 0.2s (200ms).

 

[Strima]

Before metal boards it was stipulated by the manufacturer that RCBOs must not be used on TT systems and now it’s suddenly all ok.

Wasn't that to do with them only being single pole isolation only? Some manufacturers now produce double pole RCBOs but you need to check their instructions as some state whether or not they're suitable for use with TT systems.

 

[Charlie_]

Yes that was the case and is the one I’m referring to, they have now backtracked on that and have stated there is no need to disconnect/switch the neutral ; this has also been backed up in the regs..
A lot of the regs is written to accommodate the needs of the manufacturers

 

[westward10]

There has never been a requirement for a circuit protective device to disconnect the neutral in the event of a fault on a TT system, just that it disconnects the circuit.

 

[Charlie_]

There was and is a requirement to disconnect all live conductors for a circuit protected by an RCD

And now they have rigged the rejigged the regs to allow single pole RCBOs