Two Rcd's On One Circuit

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paul8f

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Originally added to old discussion;

http://talk.electricianforum.co.uk/topic/17562-rcds-in-series/

Split off as new question due to age and length of old thread.

Doc H.

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Hi all. Just joined this forum so take it easy on the newbie....!

Hope nobody minds if I re-open this old can of worms. I'm in the middle of installing a bathroom fan (220v, ceiling mounted) in my house. Since the bathroom is the most dangerous room in the house to receive a shock, I was about to wire the fan up through an unswitched 3A spur outlet with RCD built-in, along with a 3 pole isolation switch. All the lights and sockets in the house are protected by the one RCD, and I thought there was no problem with wiring up another RCD for the fan..... that was until I stumbled on this discussion.

Does anyone know if there has been an update to the UK regulations since this discussion finished? I'm in Ireland by the way, and must try get my hands on the latest regulations here. Also, the earthing system here is mostly TN-C-S, if that makes any difference.

Anyway, I wondering if people's opposition to having two RCDs in series might be somehow down to the phenomenom of "bouncing contacts" within the RCD itself? No switch is ideal, and when the contacts open in the RCD during a Live/Neutral imbalance, the circuit will make-and-break a few times before it finally stays in the open position (all happening in the fraction of a milli-second). I suppose the worst case scenario would be when two identical RCDs get connected in series. Here, the trip times of each RCD will be very close. When an insulation problem occurs (or when someone touches a live wire), the two RCDs will be in a race with each other. Current Ib induced from the toroidal core of each RCD will begin to activate the magnetic switches. As soon as the "leading RCD" contacts begin to open, the flux on the toroidal core of the slower "trailing RCD" will be affected. Could this result in a sort of "contagious flip-flopping" of contacts, and end up affecting the overall trip time (i.e.  two RCDs, each with an average trip time of 28mS, could end up taking 34mS before one of them finally overcomes the contact bouncing to stay in the open position)?

If this is the case, a workaround might be to ensure you never install two similar RCDs in series. If RCD1 trips at an average leakage current of 29mA after 27mS make sure that RCD2 trips at an average of 20mA after 17mS. That way if a fault ever occurs downstream of RCD2, then RCD2 should trip before RCD1 in 99% of cases. Bouncing of the contacts would be minimised, and the overall trip time would be equal to (or marginally longer than RCD2). If RCD2 ever fails, RCD1 will still give you 30mA protection.

Overall my logic is telling me that in most systems the RCD is one of the weaker links due to their high failure rate. So having two in series will add redundancy and increase safety. If one doesn't trip, the other one will. My background is Electronics, so please forgive my ignorance of the figures quoted in my examples! I'd be interested to see if anybody agrees or disagrees with my theory of bouncing RCD contacts.

(...as a side note... if RCDs in series is such a no-no, then why do so many DIY stores sell extension leads with RCD protection built into the plugtop???)

 
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Welcome to the forum, I have moved your post to start a new question rather than extending a discussion that is over two years old.

To answer the last part of your question DIY stores sell leads with built in RCD's as there are still thousands of properties without any fixed RCD protection built into the consumer unit or final circuits.(RCD spurs or RCD sockets), So they still have their uses. All that will happen in your proposed scenario is that if or when a fault occurs you will have no idea which RCD will trip first. There is no regulation saying you cannot install two RCD's in series any more than there is a regulation saying you cannot have two 5A plug top fuses in series. Not uncommon for a home owner to have two extension leads connected to together round the back of the furniture to supply all the computer or TV or sky box kit. But as with RCD's it will be pot luck which one blows first should there be a fault. I would suggest that the garden and outdoor supplies are far more dangerous than the bathroom. Also I imagine your fan is double insulated with no exposed metal parts? and I doubt anyone using the bathroom is going to spend much holding the fan whilst also touching a metallic bath pipe, as such I would consider two RCD's in series to be a waste of time, effort and money installing when the circuit is already RCD protected.

Doc H.

 
Thanks Doc,

I didn't realise so many homes were still unprotected regarding RCDs. I suppose there are still a lot of older houses yet to be re-wired.

The garden and outdoor supplies are more exposed to physical cable damage, but within the house I think you are more likely to get a nastier shock if you are standing barefoot on a damp floor....! The house I moved into is old, I can't afford a re-wire right now, so I'm trying to make the fan install as safe as I can get it. The old RCD on the fuse board looks well tired, so I think I'm just going to replace that one (instead of wiring in a 2nd one for the fan).

And yes, the fan is a double insulated Xpelair unit with no exposed metal parts, and a grill that I can't even squeeze my pinkie into. Thanks for the help again.

 
Thanks Doc,

I didn't realise so many homes were still unprotected regarding RCDs. I suppose there are still a lot of older houses yet to be re-wired.
The requirement to protect all bathroom circuits was only introduced into the 17th edition of BS7671 in 2008. (701.411.3.3). Prior to that the emphasis was on supplementary equipotential bonding, 16th edition section 601. Bonding of the bathroom accessories is allowed to be omitted in 17th if RCD protection is in place. So any houses that are older than 6 or 7 years will be functioning quite safely, in compliance with regulations that were applicable when they were installed without any bathroom RCD protection. And as PVC cable can have a lifespan 60+ years under normal loading, there will quite probably be many properties without full RCD protection for several years to come. 

Bare foot on damp floors adds negligible risk of shock unless the floor is conductive introducing an earth potential. I would guess that the majority of domestic bathrooms tend to be wooden floors. As I suspected your fan is double insulated and you will need a tool to remove the cover and if it is 230v it will be installed outside of the zones, (if inside the zones it will be a 12v device), and you are unlikely to be doing any fan maintenance work with wet bare feet, so the risk of shock extremely unlikely compared to external wiring. Which is why the requirement to protect sockets used outdoors by RCD's has been in BS7671 for much longer.

It sounds to me as though what you actually need is a proper inspection and test of the circuits and the protective devices. Just because something looks 'well tired' does not mean it is not functioning correctly. How long ago did you move in? BS7671 recommends a formal inspection once every 10 years or change of occupancy, this would give you a true picture of the condition of your installation how safe it is and if it actually needs any remedial work. Again there are thousands of houses with cables that 'look old' but are perfectly capable of working safely and as the old phrase goes, 'it is isn't broke don't fix it'. An awful lot of unnecessary electrical work is done every year by people thinking things are hazardous without actually testing them first.

Doc H.

 
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I have just had a heated debate with an Architect who told me a 3036 board had to be changed as it was 'illegal'...........

architect [n] from the greek   Arch      meaning    Bloke       and Techt        Meaning        Knows nothing

 very similar to the etymology of the word Archaeologist.......

 from the greek   Arch      meaning    Bloke       and    Ology       meaning      guessing wildly and stabbing in the dark.    See     'Time Team ( ref Sweaty Phil and mincing Tony)

 
Yeah, allowing for the bonding of accessories to be omitted is understandable if the bathroom has RCD protection. Coming back to my point about two RCDs ending up in a "race condition" during an earth leakage event..... Do you think there's any truth in the trip time being increased if two identical RCDs are connected in series? I'm not an electrician, but I briefly studied RCD mechanisms during my Electronics course. I remember someone speaking out about using RCDs together, but their exact point escapes me now. Regardless, as you stated, there is nothing in the regulations banning such a setup.

The bathroom in my house, is downstairs, and is a concrete floor covered with lino. I was thinking that the path to earth in this case would be less than if the bathroom was upstairs with a carpet covered floor. Regardless, my bathroom is RCD protected and for cleaning/maintenance I will have the 3-pole isolation switch available. I still havn't started the job (not feeling too enthuastic lately... waiting for my pollen allergy to fade away.... summer sucks!). I took the fan out of the box just now. Thankfully it didn't need a special tool to open up. It's the DX4T model, and a flat head screwdriver is all I needed to remove the cover.

I moved in 6 or 7 months ago. It's an old cottage, that I can't afford to renovate yet. I take your point about getting an inspection done. How much would such an inspection set me back do you think? Or maybe just an insulation test would tell a lot about the condition of the wiring. (I've already gone through the whole house, and checked all connections for tightness). The high load items such as the shower and cooker in the extension have cable that's newer than the older body of the house, so at least that's something..

 
.... I forgot to add that the 230v fan is going to be mounted in Zone 2. I'm guessing that the UK uses a similar zoning system to Ireland., where zone 1 is the bathtub/shower area.

 
I took the fan out of the box just now.
Daft question (some may say)....

But have you read what the manufactures instructions say....??

the will normal state what permissible zones and protective devices you should use!

:popcorn

The cost of an inspection is proportional to the time taken....

The time taken is proportional to the size & complexity of he installtion...

more circuits = more time ...

Average domestic home is typically anywhere from half day to a full day IMHO

Rule of thumb half hour for checking per circuit + 1 hour checking fuse box & earthing.. 

But then also how well stuff is or isn't labeled can add / deduct half hour here or there!!!

:C

 
I originally had an Air-Stream fan, but it was not a timed model so I returned it. The instructions requested the use of a 3A fuse and a double pole switch. No mention of an RCD.

The fan I now have, the Dimpco/Xpelair, requests the use of a 5 amp fuse and multi-pole switch. (Is a 5 amp fuse not a bit high for a 15w fan, even if the fuse covers a high wattage bulb too???). How much current would a faulty/jammed fan draw?? I'm going with a 3A instead. Again, no instructions to use an RCD. No mention of the word "zone" either, but it does say that the unit is IPX5 rated and that it can be "...installed anywhere in a shower room or bathroom".

Anyway, about time I rolled up my sleeves and actually started the work rather than just talk about it! Thanks to all for advice. Paul.

 
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