Discuss Protection by RCD in the Electrical Forum area at ElectriciansForums.net

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neil_ev

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Hi all,

Just a quick query on RCD protection. Looking at 3.6.1 of the OSG it states that RCD protection is required for;

ii)socket outlets in domestic installations. - SO ALL SOCKETS SHOULD HAVE RCD PROTECTION? (unless specifically labeled as fridge, freezer ect...)

iii)for circuits of locations containing bath or shower. - EVEN IF THE LIGHT IN THE SHOWER IS RATES TO APPROPRIATE IP?

Also it does not mention outside lights. I'm sure these should be RCD protected?

Thanks in advance

Neil
 
P

Plonker 3

The BGB has alot more in regards to what needs to be RCD protected, not too sure which section as not got it too hand. But if you look through it you will see there is a lot more than just what you have seen on the OSG in regards to it.
 
S

StuSpiers

It's a general rule of thumb now in domestic situation that all circuits require RCD 30ma protection.

There are exceptions however.
 

malcolmsanford

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Hi all,

Just a quick query on RCD protection. Looking at 3.6.1 of the OSG it states that RCD protection is required for;

ii)socket outlets in domestic installations. - SO ALL SOCKETS SHOULD HAVE RCD PROTECTION? (unless specifically labeled as fridge, freezer ect...) Yes

iii)for circuits of locations containing bath or shower. - EVEN IF THE LIGHT IN THE SHOWER IS RATES TO APPROPRIATE IP? Yes

Also it does not mention outside lights. I'm sure these should be RCD protected? Not always

Thanks in advance

Neil
Next..................................................
 
D

Deleted member 26818

The Regulations require 30mA RCD protection be provided for:
(i) all 'low voltage' circuits of a location containing a bath or shower,
(ii) circuits supplying socket-outlets with a rated current up to and including 32A in Agricultural/Horticultural instllations,
(iii) socket-outlets rated up to and including 20A intended for general use by ordinary persons,
(iv) mobile equipment rated up to and including 32A used outdoors,
(v) cables concealled in walls in prescribed zones at a depth less than 50mm, that are not provided with another acceptable means of additional protection.

(i) This requirement applies to all 'low voltage' circuits, lights, showers, heated towel rails etc. and the RCD should be at the origin of the circuit, not part way along it.
(ii) Not really applicable in most circumstances, and doesn't apply to buildings used for residential purposes.
(iii) Not required for socket-outlets intended for specific items of equipment or appliances (such as boilers, washing machines, etc.), or where the socket-outlets are intended to be used by skilled or instructed persons, or their use is supervised by such persons. The exception allowing specifically labled socket-outlets is superflous, as the requirement only applies to socket-outlets intended for general use.
(iv) Only applies to mobile equipment, not items of equipment such as fixed lighting or air conditioning units.
(v) Other methods of additional protection would be use of a cable that incorporates an earthed metallic sheath such as SWA, use of earthed metallic conduit or trunking, provision of mechanical protection sufficient to prevent penetration by nails or screws or by burying the cables at a depth greater than 50mm.
There is no requirement to provide 30mA RCD protection for circuits other than those of locations containing baths or showers and those in Agricultural/Horticultural installations, althou in many instances it is the simplest and often cheapest method.
 
P

Plonker 3

The Regulations require 30mA RCD protection be provided for:
(i) all 'low voltage' circuits of a location containing a bath or shower,
(ii) circuits supplying socket-outlets with a rated current up to and including 32A in Agricultural/Horticultural instllations,
(iii) socket-outlets rated up to and including 20A intended for general use by ordinary persons,
(iv) mobile equipment rated up to and including 32A used outdoors,
(v) cables concealled in walls in prescribed zones at a depth less than 50mm, that are not provided with another acceptable means of additional protection.

(i) This requirement applies to all 'low voltage' circuits, lights, showers, heated towel rails etc. and the RCD should be at the origin of the circuit, not part way along it.
(ii) Not really applicable in most circumstances, and doesn't apply to buildings used for residential purposes.
(iii) Not required for socket-outlets intended for specific items of equipment or appliances (such as boilers, washing machines, etc.), or where the socket-outlets are intended to be used by skilled or instructed persons, or their use is supervised by such persons. The exception allowing specifically labled socket-outlets is superflous, as the requirement only applies to socket-outlets intended for general use.
(iv) Only applies to mobile equipment, not items of equipment such as fixed lighting or air conditioning units.
(v) Other methods of additional protection would be use of a cable that incorporates an earthed metallic sheath such as SWA, use of earthed metallic conduit or trunking, provision of mechanical protection sufficient to prevent penetration by nails or screws or by burying the cables at a depth greater than 50mm.
There is no requirement to provide 30mA RCD protection for circuits other than those of locations containing baths or showers and those in Agricultural/Horticultural installations, althou in many instances it is the simplest and often cheapest method.
So does this mean I couldn't use a RCD spur to extend or alter a circuit then?
 

telectrix

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i think that particular 1 relates to bathrooms, but stand to be corrected if wrong.
 
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Plonker 3

Ahh yes i see now. Seems a little silly really, can't see how it is any different weather it is at the origin of the circuit or outside the bathroom zones futher along trhe circuit. After all how many old BS3036 boards can you get RCDs for, so installing a new DB is the only solution which means a lot more expense.
 

spark 68

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Ahh yes i see now. Seems a little silly really, can't see how it is any different weather it is at the origin of the circuit or outside the bathroom zones futher along trhe circuit. After all how many old BS3036 boards can you get RCDs for, so installing a new DB is the only solution which means a lot more expense.
It matters because it is possible to get a fault before the RCD, then the RCD won't 'see' or detect this fault and will in fact allow the importing of the fault into the special location.
 

malcolmsanford

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That is spin,s interpretation of it for reg 701.410.3 though I personally don't think that is correct as per the regs, I do agree with him that it is always best practice to try and have the RCD protection at the origin.

The reg is worded as

Additional protection shall be provided for all low voltage circuits of the location of the location, by the use of one or more RCDs having the characteristics specified in regulation 415.1.1

Think Spins argument is the term "circuits" as to mean the entire circuit and so therefore fitting say a spur RCD into a lighting loop before it enters the bathroom will not comply. I would have no problem with that interpretation at all.

As said it is open to interpretation as I believe most schemes would allow you to use a RCD spur in the part of the circuit entering the bathroom, and so again there is ambiguity. The scheme may have changed their stance on this, but I still would prefer to have the whole circuit on an RCD rather than just part of it.

A lot like a Ring final, I could never understand the logic behind installing a double socket as a spur, giving that RCD protection either by buried cable or the socket itself and having the other how many sockets on the ring not. So I always tried to fit RCD protection for the entire circuit
 
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P

Plonker 3

It matters because it is possible to get a fault before the RCD, then the RCD won't 'see' or detect this fault and will in fact allow the importing of the fault into the special location.
But what about alterations where you are only responsible for the install you carry out.
 

spark 68

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But what about alterations where you are only responsible for the install you carry out.
True, but I can see spin's point of view, It better to provide RCD protection at the origin for the whole circuit, this then rules out any chance of a fault occuring prior to the RCD.
 

malcolmsanford

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Aslo sparks if you look at the definition of a "circuit" in the regs it says

An assembly of electrical equipment supplied from the same origin and protected against overcurrent by the same protective device(s)

It is the term "origin" that is key here I think.

I'm not sure how valid the argument about leakage back into the circuit down stream of the RCD is, I suppose in theory it could happen, but as CPC's are commoned in the CU there is a possibility that a non RCD protected fault on another circuit could leak fault current back into the bathroom via the CPC and any exposed conductive parts.
 

spark 68

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I was thinking more of a fault occuring upstream on the same circuit supplying the RCD Malcom, as the RCD would not be able to detect such a fault, and potentially allow the fault into the bathroom.
 
P

Plonker 3

But should you carry out a alteration you have to check Zs values so that should comply which will operate any OCPD downstream of the circuit surely?
 

spark 68

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But should you carry out a alteration you have to check Zs values so that should comply which will operate any OCPD downstream of the circuit surely?
What is to say a fault cannot occur after the alteration ?

Iam not talking about what if's here Dillib, just what is good practice and about minimising the risks.
I was merely pointing out why it matters where the RCD is placed, and my reasoning behind it.:)
 
D

Deleted member 26818

Aslo sparks if you look at the definition of a "circuit" in the regs it says

An assembly of electrical equipment supplied from the same origin and protected against overcurrent by the same protective device(s)

It is the term "origin" that is key here I think.

I'm not sure how valid the argument about leakage back into the circuit down stream of the RCD is, I suppose in theory it could happen, but as CPC's are commoned in the CU there is a possibility that a non RCD protected fault on another circuit could leak fault current back into the bathroom via the CPC and any exposed conductive parts.
In the event of an earth fault anywhere in an installation, all CPCs and earth/bonding (and in the case of TN-C-S neutral) conductors will rise to 230V potential.
Therefore any earth fault will allow fault current to bypass any RCD irrespective of where the RCD is positioned.
However as long as all Main Protective Bonding is in place and the main earth conductor is not compromised, there should be little danger.
Where an RCD is positioned part way along a conductor, an earth fault between the origin and the RCD will allow fault current to bypass the RCD, not in itself a particular danger.
However if the conductor's CPC is damaged/disconnected either due to the fault or some other reason, the fault current will have no other route other than to bypass the RCD and enter the location. This is where there is a danger.
As for the origin of a circuit, guidance is provided in BS7671 in Appendix 15, where it states that the origin is at a DB.
 

HandySparks

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In the event of an earth fault anywhere in an installation, all CPCs and earth/bonding (and in the case of TN-C-S neutral) conductors will rise to 230V potential.
I don't think so.
Surely, relative to "ground" (transformer star point), the voltage on the CPC at a line to earth fault will depend on the impedance of the fault itself (which may be close to zero) and the relative impedance of the line and earth parts of the fault circuit? Unless there is no external earth connection, it'll never rise to the supply voltage.
 

telectrix

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as i understand it, a line/earth fault will cause the potential of the earthed component to rise to line value, at the point of the fault, for the duration of the fault untill the OCPD activates. obviously, the resistance of the fault path will affect the duration of the fault.
 

HandySparks

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Hi Tel.
I quite agree that the duration of the fault is set by the resistance of the fault path and the characteristics of the OCPD. Hence why we measure Zs.

Now, "a line/earth fault will cause the potential of the earthed component to rise to line value, at the point of the fault".

Well, yes, except that a line/earth fault also drags down the line voltage at the point of the fault; so the local line voltage is reduced. So, say for example, the line and earth parts of the fault path are of equal resistance, then the voltage at the fault will be half the supply voltage. In effect, the resistance of the two parts of the fault path form a potential divider across the supply.

Or am I misunderstanding?
 
D

Deleted member 26818

In a healthy circuit, the line and neutral have similar if not equal resistance.
Would this combined resistance halve the supply voltage?
In many instances, the CPC has a smaller CSA than the live conductors, as such some volt drop would be expected, but not by a significant amount.
In many cases where an earth fault occurs, the neutral is still connected.
In such cases, the CPC would become a parallel conductor with the neutral, as such reducing resistance.
 
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