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Topic Title: 60204 Earth Leakage
Topic Summary: Requirements where leakage is greater than 10ma.
Created On: 10 October 2018 10:16 AM
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 10 October 2018 10:16 AM
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lesspiteri

Posts: 21
Joined: 14 March 2010

Hi Folks

Section 8.2.8 states requirements where earth leakage is greater than 10ma. One is "automatic disconnection of supply in case of loss of continuity of protective conductor".

I have three questions regarding this:
1. Why would this be required if as somebody suggested to me "use a 100ma RCD on the incoming supply to the machine".
2. The 100ma RCBO would be fixed wiring from a DB and therefore be BS7671. When is the 100ma acceptable i.e. a fixed piece of machinery?
3. And where on the machine would a trip device for earth continuity i.e. at the furthest point for the incoming installation earth?

Thanks
 10 October 2018 11:27 AM
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gkenyon

Posts: 5257
Joined: 06 May 2002

Answer to Question 1:

When the protective conductor breaks, the path to earth is via anyone touching exposed-conductive-parts. A 100 mA RCD would not detect a break in the incoming protective conductor, until someone touched an exposed-conductive-part. Earth monitoring is, I understand, what BS EN 60204-1 is looking for.

Whilst a 30 mA RCD may provide additional protection and might save someone's life, a 100 mA RCD might not.

BS EN 61140 (IEC 61140) is the reference standard for electrical safety and it recommends additional measures where protective conductor currents are concerned. Section 8 of BS EN 60204-1 (IEC 60204-1) aligns with this.


Answer to Question 2:
BS 7671 has its own requirements for circuits with high protective conductor currents, regardless of BS EN 60204-1 - these requirements apply to the supply to the Machinery, but BS EN 60204-1 applies to the machinery itself.

Under BS 7671, only 30 mA RCDs are considered for Additional Protection. The requirements for equipment with high protective conductor currents are covered in Regulation Group 543.7 and you will see they are similar in every respect to BS EN 60204-1, escept they are from a "supply" perspective rather than an "equipment" perspective.

Note also that if you supply the machinery from a socket-outlet, then Regulation 411.3.3 will require a 30 mA RCD, which is unlikely to be compatible with your high protective conductor current. In 17th Edition (BS 7671:2008+A3:2015), all socket-outlets rated up to and including 20 A should have an RCD, although certain exceptions are permitted. In the 18th Edition (BS 7671:2018), all socket-outlets rated upto and including 32 A should have a 30 mA RCD, the only exception being where you have a risk assessment stating the RCD is unnecessary.

Answer to Question 3:
As I said above, I think you'd be talking about an earth monitoring device, that removed the incoming supply to the machine - including the supply to the source of the protective conductor current - and not a 100 mA RCD? The RCD wouldn't trip if the incoming protective earth breaks.

To be honest, is it not easier to provide the additional earthing and bonding in accordance with Section 8 of BS EN 60204-1 and 543.7 of BS 7671?


And finally - a question from me.

What is causing the high protective conductor current? There are possibly ways to reduce it well below dangerous levels.

-------------------------
EUR ING Graham Kenyon CEng MIET TechIOSH
G Kenyon Technology Ltd

Web-Site: www.gkenyontech.com
 10 October 2018 11:42 AM
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wliptrot

Posts: 120
Joined: 30 June 2003

Who is the designer of the Fixed Wiring here?

The 8.2.8 you mention says "one or more of the following conditions for the associated protective bonding circuit shall be satisfied".

I would recommend you read the equipment manufacturers instruction and confirm their requirements. You should consider the
8.2.8 a) the protective conductor shall have a cross-sectional area of at least 10 mm2 Cu
as being potentially the best way forward.
 10 October 2018 08:42 PM
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FiftyHertz

Posts: 20
Joined: 13 April 2018

Interestingly I was reading about this today and im unsure as to why equipment with a high earth leakage needs a dual earth. Surely on any peice of equipment is the cpc is compromised then the the route to earth is you. The only thing I can think is that the leakage say 10ma may be on a metal socket per say, however at the same time surely the potential difference between the exposed metal and earth is 0 volts?

Can anyone clear this up?
Cheers
 10 October 2018 09:01 PM
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lesspiteri

Posts: 21
Joined: 14 March 2010

I believe the high protective currents are caused by motor drive filters, there are several of these on the machine and therefore a large leakage current and therefore cannot be reduced practically in this case.

There is an industrial 32A socket from the fixed wiring that the machine uses, I was aware that socket outlets were required to have RCD protection however as this was industrial socket outlet for 32A I thought it was OK. I will read up on 411.3.3 but it seems Graham you are suggesting that with 18th edition the 32A would need the RCD anyway.

So my options seem to be as follows:
1. So as per both suggestions above I could simply provide additional earthing and bonding.
2. Fit an earth monitoring device.
3. Could an alternative option not to simply wire the machine directly from the DB rather than using a socket then an RCD is not required.
 11 October 2018 12:05 AM
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mapj1

Posts: 11552
Joined: 22 July 2004

Rather than get hung up on the detail of socket size (I see the new reg boosting sales of 63A sockets to be wired to 20A breakers myself, which was probably not the intention) consider why we need this.
The idea is that no single fault should be dangerous. At least double fault to danger, ideally more 'defence in depth'

A single CPC, if it fails open circuit after someone trips over the lead, makes kit dangerous if it has high leakage, as it can deliver a lethal shock with no further fault present. In the case of a normal appliance with perhaps 3mA of leakage, then a failed CPC makes the case look live to a meter, and a bit tingly but non-lethal.
So to return to our desired condition of double fault to danger, either -

1) We must detect that open circuit, so that both the CPC and the open circuit detector and alarm must independently fail to get a dangerous state (2 faults)

2) have in effect more than one INDEPENDENT CPC path, ( again now 2 faults to danger) Pragmatically these 2 paths can be combined after any MET where it is all 10mm or larger, as you will struggle to break that by tripping over it, and accidental removal is unlikley.

3) Hard wire it in a way that the interrupted CPC is an incredible fault. (conduit or something.)


The easy one is usually to loop up the CPC so it is X-bonded to the CPC of another circuit, and both would have to be off earth for danger to occur.
Arguably SWA armour with one core acting as CPC are also capable of being seen as independent earth paths, even though you have to cut one to attack the other.

Coming back to the socket, I think if you left it at 32A, had a bit of robust GY and some wingnut arrangement, and a label saying ' socket for high leakage equipment XYZ , NO RCD do not operate without this earth bond' or similar words, that would meet the essential requirements.

-------------------------
regards Mike
 12 October 2018 01:27 PM
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AJJewsbury

Posts: 17435
Joined: 13 August 2003

The only thing I can think is that the leakage say 10ma may be on a metal socket per say, however at the same time surely the potential difference between the exposed metal and earth is 0 volts?

The leakage current comes from the line conductor(s) - via some kind of impedance that limits the current. If c.p.c. is broken, any exposed metalwork connected to the appliance side of the c.p.c. break is then not connected to earth, but is still connected to line (via that impedance). It therefore is pulled up to towards line voltage - so isn't 0V w.r.t. earth.

- Andy.
 12 October 2018 01:36 PM
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mapj1

Posts: 11552
Joined: 22 July 2004

The usual culprit is small value capacitance betwen the phases (and the neutral too if supplied) and the chassis, as part of interfernce filtering that is essential for EMC reasons. This is a near constant current source from phsae voltage and the chassis will be pulled up to an unsatisfactory potential nearer the phase voltage if the CPC resistance is too high, or worse it is removed.

-------------------------
regards Mike


Edited: 12 October 2018 at 01:48 PM by mapj1
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