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Topic Title: Inspection and Testing of installations
Topic Summary: Testing beyond the spur!
Created On: 22 February 2010 07:51 PM
Status: Post and Reply
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 24 February 2010 05:34 PM
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AJJewsbury

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What, as in 3 separate FCUs?

No, as in a group of three fuses supplying a single 3-phase circuit. The definition applies to all situations, not just single-phase rings.
- Andy.
 24 February 2010 05:34 PM
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spinlondon

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Originally posted by: OMS
If a circuit starts at an overcurrent device, why place an 's' in brackets? Surely according to you each overcurrent device would be the start of a new circuit.


Not really no - that just allows you to use seperate devices for overload and fault protection at differing points in the same circuit.

Take a motor circuit - fault protection is usually offered by the CPD at the circuit source end but overload protection is usually offered at the circuit load end. This complies with BS 7671 and is a single circuit by definition - thats why we have a (s) in teh definition

OMS


Usually offered at the circuit source?
Isn't the circuit source the same as the circuit origin, which is where the overcurrent device is positioned?
 24 February 2010 05:38 PM
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spinlondon

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Originally posted by: AJJewsbury
What, as in 3 separate FCUs?

No, as in a group of three fuses supplying a single 3-phase circuit. The definition applies to all situations, not just single-phase rings.
- Andy.

Do they do FCUs for 3 phase then?
Do fuses offer overload, fault, overcurrent protection or all three?
 24 February 2010 05:40 PM
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OMS

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Originally posted by: spinlondon

Originally posted by: OMS

I'm quite happy to use BS7671 when making my opinion


So what is your opinion on my question then Spin

Regards

OMS


Quite simple really, specify the correct size of cable to allow for Zs to be within acceptable limits when designing the circuit.


Can't disagree with that - my question was perhaps based on how you would consider the situation if it was encounterd on a PIR

OMS

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Failure is always an option
 24 February 2010 05:44 PM
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OMS

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Originally posted by: spinlondon

Originally posted by: AJJewsbury

If a circuit starts at an overcurrent device, why place an 's' in brackets?


'cos 3-phase circuits need more than a single fuse?

- Andy.


What, as in 3 separate FCUs?


Well you used the fact that the definition allows overcurrent devices to be singular or plural Spin - Andy and I have given you reasons why that's the case

What's that got to do with 3 fused spurs I fail to understand

OMS

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Failure is always an option
 24 February 2010 05:49 PM
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spinlondon

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Originally posted by: OMS
Originally posted by: spinlondon
Originally posted by: AJJewsbury
If a circuit starts at an overcurrent device, why place an 's' in brackets?

'cos 3-phase circuits need more than a single fuse?

- Andy.

What, as in 3 separate FCUs?


Well you used the fact that the definition allows overcurrent devices to be singular or plural Spin - Andy and I have given you reasons why that's the case

What's that got to do with 3 fused spurs I fail to understand

OMS


Did you not state that an FCU could be the origin of a circuit?
As I am not aware that there are such animals as 3phase FCUs, I assume the origin of a 3phase circuit could be 3 FCUs, one for each phase.
 24 February 2010 05:50 PM
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OMS

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Usually offered at the circuit source?

Isn't the circuit source the same as the circuit origin, which is where the overcurrent device is positioned?


Essentially yes - although in my motor circuit I have more than one overcurrent device undertaking different functions - one at the source and one at the load so it still meets the definition of being a single circuit.

In the FCU protecting the spur off the ring, the 13A device may well be providing both overcurrent and fault protection so clearly it is the origin of a new circuit.

Regards

OMS

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Failure is always an option
 24 February 2010 05:52 PM
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spinlondon

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Of course none of this gets us away from the fact, that Appendix 15 clearly states that the origin of a circuit is at the DB, and just why we should ignore that information.
Perhaps I should rephrase that to the origin of a final circuit.
 24 February 2010 05:56 PM
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OMS

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Did you not state that an FCU could be the origin of a circuit?


I did - it meets the relevant definition


As I am not aware that there are such animals as 3phase FCUs, I assume the origin of a 3phase circuit could be 3 FCUs, one for each phase.


Don't follow you Spin - but if you are asking me can a 3 phase circuit be protected by 3 individual fuses (one in each phase) then yes. Could you do that via 3 fused spurs then assuming they were unswitched then essentially yes

You appeared to determine that the definition of circuit containing the words protective device(s) had some specific meaning.

Andy and I simply pointed out that a polyphase circuit would have more than one protective device as would a circuit where different overcurrent functions wrere undertaken by seperate devices - all of which would still meet the definition of circuit

OMS

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Failure is always an option
 24 February 2010 05:59 PM
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spinlondon

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Originally posted by: OMS
Usually offered at the circuit source?

Isn't the circuit source the same as the circuit origin, which is where the overcurrent device is positioned?


Essentially yes - although in my motor circuit I have more than one overcurrent device undertaking different functions - one at the source and one at the load so it still meets the definition of being a single circuit.

In the FCU protecting the spur off the ring, the 13A device may well be providing both overcurrent and fault protection so clearly it is the origin of a new circuit.

Regards

OMS


I'm certain that someone stated that circuits start at the overcurrent device, now you seem to be stating that there can be more than one overcurrent device in a circuit.
Surely if the origin is at an overcurrent device, it stands to reason that there can not be 2 such devices in one circuit. It must be 2 circuits.
I tought the requirement was for overload and fault protection?
Can we dispense with overload protection if we have overcurrent protection instead?
 24 February 2010 06:00 PM
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OMS

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Originally posted by: spinlondon

Of course none of this gets us away from the fact, that Appendix 15 clearly states that the origin of a circuit is at the DB, and just why we should ignore that information.

Perhaps I should rephrase that to the origin of a final circuit.


No one suggested ignoring it - it was in fact part of my original post to the OP (ie if the characteristics of the ring CPD protect the spur then you can ignore the spur as being the commencement of another circuit

If it doesn't protect that spur however (and as I asked you ) you could reference to the 13A fuse and treat the spur as if it was a new circuit

I notice you won't answer the question Spin.

Regards

OMS

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Failure is always an option
 24 February 2010 06:05 PM
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spinlondon

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Originally posted by: OMS
Originally posted by: spinlondon

Of course none of this gets us away from the fact, that Appendix 15 clearly states that the origin of a circuit is at the DB, and just why we should ignore that information.

Perhaps I should rephrase that to the origin of a final circuit.


No one suggested ignoring it - it was in fact part of my original post to the OP (ie if the characteristics of the ring CPD protect the spur then you can ignore the spur as being the commencement of another circuit

If it doesn't protect that spur however (and as I asked you ) you could reference to the 13A fuse and treat the spur as if it was a new circuit

I notice you won't answer the question Spin.

Regards

OMS


I thought I had, and that you stated that you couldn't disagree with my approach.
Of course you could always follow the advice and appropriate Regulations in BS7671 concerning such an occurance.
 24 February 2010 06:12 PM
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OMS

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I'm certain that someone stated that circuits start at the overcurrent device, now you seem to be stating that there can be more than one overcurrent device in a circuit.


There can Spin - think three phase

Surely if the origin is at an overcurrent device, it stands to reason that there can not be 2 such devices in one circuit. It must be 2 circuits.


Or you consider that neither device provides the complete function of overcurrent so it is only collectively that they act to create a circuit.

Let me ask this - if it insert an RCD into a circuit (say in an enclosure close to a DB) have I now created a circuit between the MCB and teh RCD and a further circuit between the RCD and the load - I suggest not

I tought the requirement was for overload and fault protection?
Can we dispense with overload protection if we have overcurrent protection instead?


You may but that's a different argument it;s not always required to provide overload protection to a circuit

Overcurrent is any current that exceeds a rated value

Overload current is Overcurrent in an electrically sound circuit

Fault current is Overcurrent resulting from a fault

Overcurrent protective device is used for a device that provides protection from either fault current, overload current or both. It need not protect from both and may be more than one device fulfilling different functions

OMS

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 24 February 2010 06:19 PM
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OMS

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I thought I had, and that you stated that you couldn't disagree with my approach.


Of course you could always follow the advice and appropriate Regulations in BS7671 concerning such an occurance.


Indeed, as you decided to couch a reply in terms of how you would design for such a situationwith which I agree - however I note the silence is deafening when it comes to how you would code such a situation if encountered on a PIR.

Would you care to point out to me the appropriate regulations in BS7671 then Spin so I could follow the advice and guidance

OMS

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 24 February 2010 07:15 PM
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spinlondon

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I tink OMS, you should look them up yourself.
You can't expect other people to do your work for you.
Getting back to the origin of circuits, I still don't understand why we should disregard either the information provided in Appendix 15, or the requirements of Regulation 314.4, in favour of a definition which does not state where the origin of acircuit is
 24 February 2010 07:40 PM
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OMS

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I tink OMS, you should look them up yourself.


I have and it leads me to believe that you could either determine that the socket outlet circuit fails to comply with BS 7671 because the limiting Zs value isn't achieved by circuit design - your stance I believe.

Or you could determine that the circuit actually complies with BS 7671 because the limiting Zs and short circuit protection of conductors is now referenced to the 13A fuse - ie it is a seperate circuit with it's own overcurrent protective device - my stance essentially.

You can't expect other people to do your work for you.


LoL - I'm a consultant Spin - that's what I do

Getting back to the origin of circuits, I still don't understand why we should disregard either the information provided in Appendix 15, or the requirements of Regulation 314.4, in favour of a definition which does not state where the origin of acircuit is


Avoiding the question more like it Spin


No one said to disregard it Spin - just that the appendix shouldn't be used to determine a definition of a circuit in favour or Part 2 - neither should a regulation that is dealing with a specific aspect be used to determine the definition of a circuit - again that's in Part 2.

Regards

OMS

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 24 February 2010 08:01 PM
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spinlondon

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In answer to your question, regarding the high Zs,
just apply the recomendations of Regulation 531.3.1.

The Appendix is not defining anything, it informs where the origin of a final circuit is.
It shows the arrangement of ring and radial final circuits, with and without fused spurs.
I can't see any circuit there that shows the origin to be at an FCU, however I can see circuits with spurs protected by FCUs.
As to disregarding a Regulation because it is specific?
Is it perhaps that we should disregard it because it doesn't fit in with your view point, perhaps because it specifically states that final circuits should be connected at DBs?
Of course that Regulation only specifically applies where there is more than one final circuit.
 24 February 2010 08:10 PM
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OMS

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In answer to your question, regarding the high Zs,

just apply the recomendations of Regulation 531.3.1.


LoL - well wriggled Spin - humour me and lets pretend the RCD isn't an option due to nuisance tripping.

The Appendix is not defining anything, it informs where the origin of a final circuit is.


For a specific arrangement of 2 pre designed standard circuits I believe

I can't see any circuit there that shows the origin to be at an FCU, however I can see circuits with spurs protected by FCUs.


I see if you can see circuits protected by FCU's then I guess I rest my case

QED m'lud

OMS

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 24 February 2010 08:26 PM
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spinlondon

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Amazing, I'm the wriggly one, yet you have not given a single valid reason why we should disregard the requirements and information given in BS7671.
I don't think the intention is to show just 2 predesigned standard circuits.
I think it is designed to show that the circuits can have unfused spurs, fused spurs, equipment spurred off of the circuit, fused spurs connected directly to the circuit, or spured off of the circuit a whole host of options. It doesn't show equipment connected directly to the circuit, although it does show an FCU connected directly to a circuit.
But as I stated before, it does not show any circuit starting at an FCU, whereas it does show and state that they start at DBs.
 24 February 2010 09:55 PM
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SandyBoiler

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The intention of Appendix 15 is clearly stated in the first paragraph...

"This appendix sets out options for the design of ring and radial final circuits for household and similar premises..."


...The force is strong with this one...

-------------------------
Andy B
IET » Wiring and the regulations » Inspection and Testing of installations

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