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Topic Title: Are 30mA SRCDs a bit pointless in agricultural installations?
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Created On: 28 February 2013 03:38 PM
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 28 February 2013 03:38 PM
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weirdbeard

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The reg 705.411.1 requires the circuits supplying sockets upto 32A to be 30mA protected so apart from being a double layer of RCD protection incase one fails is there any point in fitting SRCD's?

For instance there may be a circuit that is supplied by a 100mA TD RCD with a 30mA SRCD but this arrangement is not compliant with the regs, though surely must be safe?

Just wondered what others thought of this, as I realised I recently installed a non-compliant installation!
 28 February 2013 03:47 PM
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tattyinengland

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Well I'd say that the S type RCD (Not necessarilly 30mA) is for use on sub distribution circuits to other distribution boards, buildings, sheds, etc.

The 30mA RCD is then used for circuits from the DB out.

I'm sure there are more ways to do it though...

Regards
 28 February 2013 03:50 PM
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weirdbeard

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Hi Tatty, not sure if I have explained things but if it helps:

SRCD= socket with integrated RCD
 28 February 2013 03:57 PM
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tattyinengland

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My abject appologies.........I'll leave the original reply on so others don't make the same stupid mistake as I did.

I certainly don't think that 30mA SRCDs (Know what they are now are now ) are actually required if the circuit is also 30mA RCD protected. In areas of increased risk, I guess it would be more reasuring though, albeit unecessary.

Regards
 28 February 2013 04:03 PM
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weirdbeard

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Thanks for your replies Tatty, what I was thinking is you could have a light switch next to an SRCD, the wiring to the lightswitch is safe on the 100mA , but the wiring to the SRCD needs to be 30mA!
 28 February 2013 04:09 PM
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AJJewsbury

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The reg 705.411.1 requires the circuits supplying sockets upto 32A to be 30mA protected

Odd that the wording is different to 411.3.3 that only requires the socket to be protected (rather than the circuit) and (for comparison) 701.411.3.3 that requires the circuit to be protected - 705.411.1 seems only to ask that an RCD is provided (anywhere?) in the circuit?

I guess that your interpretation is correct and the authors intended the RCDs to be at the origin of the circuit, but perhaps it could be read otherwise...

- Andy.
 28 February 2013 04:21 PM
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weirdbeard

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Originally posted by: AJJewsbury



I guess that your interpretation is correct and the authors intended the RCDs to be at the origin of the circuit, but perhaps it could be read otherwise...

.


Thanks for the reply Andy, now you mention it I guess that SRCD's would incorporate an RCD in the final circuit albeit only just before the outlet!




Cheers.
 28 February 2013 04:32 PM
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OMS

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411.3.3 only requires the socket to offer protection to the flexible connection/appliance

705.411.1 as above

The risk is from the use/abuse of the equipment not protection from damage to the circuit wiring

701.411.3.3 requires the circuit to be protected because of particular risks due to being wet and naked from all potential sources of fault.

what I was thinking is you could have a light switch next to an SRCD, the wiring to the lightswitch is safe on the 100mA , but the wiring to the SRCD needs to be 30mA!


Essentially, the circuit wiring of the socket circuit would be fine on 100mA - the 30mA SRCD is protecting the user from abuse beyond the socket outlet terminals

regards

OMS

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 28 February 2013 04:45 PM
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weirdbeard

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Originally posted by: OMS



Essentially, the circuit wiring of the socket circuit would be fine on 100mA - the 30mA SRCD is protecting the user from abuse beyond the socket outlet terminals




Agreed, but the reg in question mentions the circuit being protected, rather than the sockets themselves.
 28 February 2013 04:59 PM
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OMS

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OK - I can go along with that - I guess your 30mA SRCD's are non compliant then.

regards

OMS

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 28 February 2013 11:43 PM
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sparkingchip

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I can see a RCD on the circuit is safer than a SRCD because it gives better protection from raised voltage on the CPC and anything connected to it, such as a class I appliance plugged into the SRCD, better still would be a single 30mA RCD main switch for the whole installation.

However if fitting a SRCD you can install a non-latching one giving additional protection from appliances or other equipment restarting unexpectedly if power is restored after a outage.

Andy
 01 March 2013 07:28 PM
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weirdbeard

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Originally posted by: sparkingchip

I can see a RCD on the circuit is safer than a SRCD because it gives better protection from raised voltage on the CPC and anything connected to it, such as a class I appliance plugged into the SRCD, better still would be a single 30mA RCD main switch for the whole installation.



Hi Andy, not quite with you there, if we take an example where there is a radial circuit feeding one socket outlet which is protected by a 30mA RCBO at the DB, how does this differ from where the protection for the end user of the socket is provided by a 30mA SRCD?



However if fitting a SRCD you can install a non-latching one giving additional protection from appliances or other equipment restarting unexpectedly if power is restored after a outage.




Agreed, I think there are also other advantages to SRCD's for example where an external socket might be for general use by, but where the DB is in a shed that might be locked up and not all workers have keys to access it.

It also gives easy access to the test button which can easily be checked with each use without interfering with any other supplys.

etc.

It feels a bit odd to me that the regs don't seem to allow this kind of arrangement but realistically the arrangement is safe as can be RCD-wise to the socket user?
 01 March 2013 07:40 PM
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weirdbeard

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I personnaly think these changes that came about with the 17th for RCDs on farms were perhaps led by the manufacturers influence?

If so it's a shame that the same manufacturers haven't come up with any goods suitable for upgrading existing installations, for example where there is an existing 200A switch fuse with BS88s theres no way of say adding an RCD into the tails supplying it, you can only change the whole lot for an MCCB with an earth leakage module, as far as I know - does anyone know different?
 04 March 2013 02:59 PM
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AJJewsbury

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Hi Andy, not quite with you there, if we take an example where there is a radial circuit feeding one socket outlet which is protected by a 30mA RCBO at the DB, how does this differ from where the protection for the end user of the socket is provided by a 30mA SRCD?

Consider a L-PE fault in the cabling between the socket and the DB. The MCB (or fuse) should operate, but might take longer than 40ms prescribed for additional protection - during which the c.p.c. and anything connected to it (e.g. the user of a hand-held class 1 appliance) is subjected to a raised voltage (e.g. 115V-ish on TN, perhaps closer to 230V on TT).

- Andy.
 04 March 2013 05:02 PM
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jcm256

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slightly off topic.
I know you are talking about socket-outlets with integral RCD, but the HSE frowns upon adapter type RCD for protecting HP water cleaners and steam cleaners.
What is that other one called that you cut off the plug-top and connect the flex direct to the RCD. Used to have trouble with hire firms, supplying a plug-top RCD along with a steam cleaner if hired, trying to put them on the right track, you might get, that's it not worth the bother will not hire out steam cleaners again, all for a few pounds to leave it safe for use.

In circumstances where the RCD is not provided as part of the fixed installation, the RCD should accompany the machine and should be installed so that it protects both the machine and the supply cable. The use of adapter type or plug top type RCDs is not recommended because these are often left out of use, may not be suitable for wet environments, or may be incorrectly installed

http://www.hse.gov.uk/pubns/priced/pm29.pdf
 04 March 2013 05:13 PM
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Parsley

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The updated GN7 is worth a look, it doesn't detail the exact reason why the reg 30ma RCD's on socket outlet circuits 32A< was introduced. It's probably due to wanting to disconnect the circuits that are most likely to develop faults quickly in order to prevent the expensive livestock getting zapped.

Regards
 04 March 2013 07:11 PM
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slittle

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Originally posted by: weirdbeard

I personnaly think these changes that came about with the 17th for RCDs on farms were perhaps led by the manufacturers influence?



If so it's a shame that the same manufacturers haven't come up with any goods suitable for upgrading existing installations, for example where there is an existing 200A switch fuse with BS88s theres no way of say adding an RCD into the tails supplying it, you can only change the whole lot for an MCCB with an earth leakage module, as far as I know - does anyone know different?


RCD's have been required on farms before the 17th. It looks like I managed to miss the interesting bits of this post.

I wouldn't be fitting RCD sockets in any of my sites, it would be RCD or RCBO to protect normal sockets or moisture proof depending on the application.

Stu
 04 March 2013 07:37 PM
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weirdbeard

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Originally posted by: AJJewsbury


Consider a L-PE fault in the cabling between the socket and the DB. The MCB (or fuse) should operate, but might take longer than 40ms prescribed for additional protection - during which the c.p.c. and anything connected to it (e.g. the user of a hand-held class 1 appliance) is subjected to a raised voltage (e.g. 115V-ish on TN, perhaps closer to 230V on TT).



AJ, I can see where you are coming from but wouldn't this situation also happen when there is a fault on other circuits that are not the socket circuit, for example if there was a fault on a lighting circuit it would also raise the voltage on the earth to the socket circuit?
 04 March 2013 07:40 PM
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weirdbeard

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Originally posted by: Parsley

The updated GN7 is worth a look, it doesn't detail the exact reason why the reg 30ma RCD's on socket outlet circuits 32A< was introduced. It's probably due to wanting to disconnect the circuits that are most likely to develop faults quickly in order to prevent the expensive livestock getting zapped.



Hi Parsley, wouldn't it be fair to say that in the fixed wiring a socket circuit is no more likely to develop faults than a lighting circuit? It's surely the equipment plugged into the socket thats more likely to cause the problems?
 04 March 2013 07:48 PM
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OMS

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wouldn't it be fair to say that in the fixed wiring a socket circuit is no more likely to develop faults than a lighting circuit?


On a farm ? - I suspect not just due to the geography of the relevant outlets

Regards

OMS

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Failure is always an option
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