Outside lighting circuit Question

[johnboy6083]

for max. Zs, you use the MCB at the origin of the circuit, not some fuse half way round the RFC. if you look atr the shedule of test results on a MWC, you will see that it asks for the details of the modified circuit. the Zs you are measuring is for the circuit, not just the spur from it.

I disgree tel I would class the FCU fuse as a new circuit. So another way of lookinmg at it would be to imagine the circuit that the FCU is fed from as a submain. If the VD is within spec, disconnection times are met, and the Iz of the cable is ok, then all is good.

 

[telectrix]

details of the modified circuit is for details of the circuit after it has been modified, not as it was before

 

[sambotc]

MCB 32A Type B Max Zs= 1.44ohm....

Good luck getting it that low...

Zs at the ring final is 0.50. cable is 1.5mm T&E and say 10mtrs long. Table I1 in GOSG suggests a resistance of R1 + R2 = 30.20m ohms/mtr

30.20 x 10 = 0.302 ohms? + 0.50 = 0.80 ohms

Is that correct?

I'm hoping/thinking its the switched fused spur causing the issues.

Surely disconnection times are there to ensure safe operation of the over current device under fault conditions, so providing the circuit is made safe, regardless of which device does this, the objective has been achieved?

yet another grey area in the world of electricity!

 

[sambotc]

Just to add its TNCS Ze of 0.10, so yeah definitely a fault somewhere that I will try my hardest to find.

 

[Barry Rathbone]

Your first line of protection is your 1361 fuse, NOT the 32A.... your are a couple of ohms out from a .4 disconnection time on a 3A fuse.
So check connections/cable and it wiil be fine...

 

[sambotc]

Surely with such a high resistance though, and knowing something is not right it's better to find the fault than make a fault pass by changing the fuse?

 

[glenncaine]

Rewire the ****er.

 

[pencilpusher]

Rewire the ****er.

Endearing first post.... Welcome!

 

[johnboy6083]

although i mentuioned that i would use the Zs that applies for the fuse, i would in this sityuation endeavour to find the route cause of such a high resistance connection. i do this also when doing r1, r2 and rn tests ion a ring ect. if there is a massive difference in the conductors, then i will investigate.

 

[ackbarthestar]

A circuit is defined by:
'An assembly of electrical equipment supplied from the same origin and protected from overcurrent by the same protective device/s'

A Final circuit is define as:
'A circuit connected directly to current using equipment, or a socket outlet or socket outlets or other outlet point for the connection of such equipment'

Device/s being the operative word

So the protection of the circuit is by the OCPD at the origin.

If you consider the origin to be at the consumer unit then the rfc includes both the spur and its BS1362 fuse; if on the other hand you consider the circuit to be directly connected to equipment as defined by a 'final circuit' then you would only look at the spur at the point where it starts, that is, the SFCU.
We would then record our results on our MWC or, as possibly defined, an EIC , being a new circuit.

However, we know that when measuring the R1+R2 and subsequently the Zs of a rfc we take account of any spurs, fused or unfused, and use the maximum value as our Zs.

Additionally, would a flexible cable supplied by a plug and socket clipped to a structure supplying equipment be regarded as a final circuit requiring a separate EIC?

So I must veer towards Tel's argument on this one.

 

[brman]

Sorry, I am not quite sure what you are arguing....

A circuit is defined by:
'An assembly of electrical equipment supplied from the same origin and protected from overcurrent by the same protective device/s'
so anything being supplied from a FCU is being supplied from the same origin and protected by the same PD. ie. the origin of that circuit is the FCU.

A Final circuit is define as:
'A circuit connected directly to current using equipment, or a socket outlet or socket outlets or other outlet point for the connection of such equipment'
ok. so it is a final circuit. Good so far

Device/s being the operative word
No sure what you mean here. We have a single protective device (the FCU). Your not saying all DBs have to have more than one PD are you?

So the protection of the circuit is by the OCPD at the origin.
Origin of that circuit yes. Not necessarily the origin of the installation which would of course rule out sub boards completely.

If you consider the origin to be at the consumer unit then the rfc includes both the spur and its BS1362 fuse; if on the other hand you consider the circuit to be directly connected to equipment as defined by a 'final circuit' then you would only look at the spur at the point where it starts, that is, the SFCU.
We would then record our results on our MWC or, as possibly defined, an EIC , being a new circuit.
Suits me

However, we know that when measuring the R1+R2 and subsequently the Zs of a rfc we take account of any spurs, fused or unfused, and use the maximum value as our Zs.
True, but so what? I'd argue that we do this because it would be a pain to treat every fused spur as a separate circuit, both from a design and testing point of view. However it doesn't mean we have to do it that way! If the spur meets the max Zs of MCB in the main DB then great. I am just arguing that if it doesn't we still have the FCU fuse to fall back on.

Additionally, would a flexible cable supplied by a plug and socket clipped to a structure supplying equipment be regarded as a final circuit requiring a separate EIC?
No, because the regs only deal with fixed wiring. Anything beyond a socket doesn't count

So I must veer towards Tel's argument on this one.

HOWEVER: I am only arguing the point because I am laid up with a bad back and cannot work at the moment. I can see the arguements both ways and suspect it is one of those gray areas where the regs are abiguous. So feel free to ignore me......... :lol:

 

[ackbarthestar]

How could I ignore you laid up in bed. Nothing worse than twiddling you thumbs and not being able to move around.

RFC's are peculiarly a British invention aimed to satisfy the reduction in the use of copper, and so require a peculiarly British solution of testing. I think providing a separate EIC for a fused spur to a socket being even more peculiar.....

Additionally, would a flexible cable supplied by a plug and socket clipped to a structure supplying equipment be regarded as a final circuit requiring a separate EIC?
No, because the regs only deal with fixed wiring. Anything beyond a socket doesn't count

Read more: http://www.electriciansforums.net/i...ighting-circuit-question-3.html#ixzz1zkUUcRjj
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Or a SFCU supplying a flex outlet fed to a socket?

 

[sambotc]

Surely the MCB being 32a isn't protecting the cable in question as the cable isn't rated to 32A

In a short circuit scenario then it will not matter, but if for instance the cable was supplying a fitting capable of drawing 30amps under a fault condition then the BS1362 fuse is the only thing protecting it from being damage (theoretically) therefore the fuse must be the main OCPD ?

It's all theoretical BS I know but isn't that the principle most of the reg's are written, theory?

Otherwise, why have the fuse at all?

 

[ackbarthestar]

Surely the MCB being 32a isn't protecting the cable in question as the cable isn't rated to 32A

In a short circuit scenario then it will not matter, but if for instance the cable was supplying a fitting capable of drawing 30amps under a fault condition then the BS1362 fuse is the only thing protecting it from being damage (theoretically) therefore the fuse must be the main OCPD ?

It's all theoretical BS I know but isn't that the principle most of the reg's are written, theory?

Otherwise, why have the fuse at all?

I see your point. However, what sort of load would be connected to a spur of a ring final circuit that draws 30A? Since water heaters are discouraged and fixed appliances with a loading of greater than 10A (2kW ) are also discouraged.
Its unlikely that an overcurrent of that magnitude would last long since its heading for a S/C.

 

[Guitarist]

brman......"No, because the regs only deal with fixed wiring. Anything beyond a socket doesn't count"

According to the regs, fixed wiring is still fixed wiring regardless of whether it is connected via a plug and socket or an FCU. The old get out clause of outbuildings "plugged in" doesn't work any longer


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