I understand your logic i.e. what's the difference between have a socket with a 13A extension reel going into vs a FCU with 13A fuse and long "extension" out to garage.
However difference is fixed vs non-fixed wiring. For fixed wiring then I think you already know the answer. So I would personally be looking to (depending on cost and longer term plans):
1. increase size of cable from FCU to sockets in garage, to reduce Zs to within acceptable limits.
2. or run a separate circuit for the CU to the garage with acceptable Zs for that circuit.
I know it's not ideal, but keeps you right.
However difference is fixed vs non-fixed wiring. For fixed wiring then I think you already know the answer. So I would personally be looking to (depending on cost and longer term plans):
1. increase size of cable from FCU to sockets in garage, to reduce Zs to within acceptable limits.
2. or run a separate circuit for the CU to the garage with acceptable Zs for that circuit.
I know it's not ideal, but keeps you right.
I am not sure what you are saying
Is it you have a rfc with a fcu, the ring maximum Zs is lower than its protective device 32A mcb - so 1.1 ohm.
Then you have a sub circuit from this fcu this circuit has a lower Zs than its protective device 13A fuse - so 1.84 ohm.
Where is the high Zs related to?
Is it you have a rfc with a fcu, the ring maximum Zs is lower than its protective device 32A mcb - so 1.1 ohm.
Then you have a sub circuit from this fcu this circuit has a lower Zs than its protective device 13A fuse - so 1.84 ohm.
Where is the high Zs related to?
D
davesparks
It is protected by the 13A fuse so needs to meet the Zs requirement for the 13A fuse.
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It is protected by the 13A fuse so it is fully compliant and nothing needs to be done.
Cheers @davesparks Guess I'm belt and braces type of guy.
Always thinking worst case and looking at potential changes down the line....
D
davesparks
Cheers @davesparks Guess I'm belt and braces type of guy.
What worst case are you looking at here?
And what potential change?
We cannot protect against potential changes down the line because we cannot know for certain what will, or will not happen in the future.
Agree with you fully
Definitely can't see into the future, just what I've seen happen over the years, always plays at the back on my mind e.g. FCUs swapped out for sockets, fuses in FCU replaced by screws/wire.
No matter what we try to protect against (perceived or real) something will always be missed, hence evolution of the regs over the years.
Hope that makes sense? Apologies if I've inadvertently offend you, certainly was not my intention.
D
davesparks
Whilst those things can and do happen it is still very hard to protect against them now, and certainly unfair to charge a customer for extra work to protect against something they, or a future owner of a property might do.
R
radiohead
This is where certification, (although a ball ache), protects you as well as the client. State EXACTLY what has been installed and give all the relevant information and test results. That's your insurance policy if someone subsequently make a dangerous alteration to your work.
D
davesparks
What is YOUR opinion? Or do you just follow the herd?
Need to be a little bit careful when applying this.
Yeah, this method then moves the job of earth fault protection from the OCPD to the RCD as the fault current is not enough to operate the OCPD quickly enough. The OCPD will still be covering short circuit protection so this then needs to be checked. If cpc is reduced size then probably ok as the fault current will be higher across live conductors, but you will still need to verify this is satisfactory. If you have situation where cpc is same size as live conductors and it wont satisfy Zs then it also won't satisfy live conductor thermal constraints either as will be the same fault current.
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Agree with you fully
Definitely can't see into the future, just what I've seen happen over the years, always plays at the back on my mind e.g. FCUs swapped out for sockets, fuses in FCU replaced by screws/wire.
No matter what we try to protect against (perceived or real) something will always be missed, hence evolution of the regs over the years.
Hope that makes sense? Apologies if I've inadvertently offend you, certainly was not my intention.
Seriously mate, I would not worry about what other people do in the future. If they make changes to an electrical installation then they are responsible for them.
To add , the live conductors need checking against the formula T= K2 S2/I2 , the resulting number is how long it takes for damage to the live conductors to occur, nothing to do with whether the earth fault loop impedance complies or not.
The short circuit fault loop can be measured and checked against the time characteristics of the ocpd, the given number in seconds for disconnection can then be compared to the formula above to see if the live conductors are thermally protected against short circuit faults.
I think this is what was implied?
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Great point.
Unfortunately the 'other' people are friends and family at the moment, and I'm sure you know what they are like...I have stopped recommending they employ armed security to prevent them touching anything electrical in the house. Now I recommend they just employ a butler to flick their switches for them. Problem solved.
On a more serious note, also need to make sure I reference the regs more often rather than relaying on the old brain matter, before jumping in with my comments.
Lesson(s) learned. Thanks to all.
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