High Zs

[Bailey326]

13amp FCU off ring main supplying sockets in shed.
Zs at furthest socket in shed is 1.26ohms which is too high for a 32 amp mcb.
Would you use the 13amp fuse in the FCU to meet your Zs reading?
Zs at FCU is 0.70.

 

[Let me think]

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.

 

[bill01803]

Is it not RCD protected?

 

[radiohead]

If it's 30ma RCD protected at the origin of the final circuit the maximum permitted Zs is 1667 ohms. If it's not, it's supplying sockets, why isnt it?

 

[telectrix]

it complies with the max. allowed, but not the 80% correction for 70 deg. however, will that cable ever get anywhere near 70 deg.? not a hope in hell.

 

[Julie.]

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?

 

[davesparks]

13amp FCU off ring main supplying sockets in shed.
Zs at furthest socket in shed is 1.26ohms which is too high for a 32 amp mcb.
Would you use the 13amp fuse in the FCU to meet your Zs reading?
Zs at FCU is 0.70.

It is protected by the 13A fuse so needs to meet the Zs requirement for the 13A fuse.
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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.

It is protected by the 13A fuse so it is fully compliant and nothing needs to be done.

 

[Let me think]

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....

 

[davesparks]

Cheers @davesparks Guess I'm belt and braces type of guy. Always thinking worst case and looking at potential changes down the line....

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.

 

[Midwest]

You can’t get anything bigger than a 13A BS1362 fuse, unless it’s replaced with a nail. So what else can change?

 

[Let me think]

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.

 

[davesparks]

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.

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.

 

[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.

 

[Bailey326]

Has RCD protection as well.
Most answers confirming what I thought with it being ok but have heard mixed opinions on this before.

 

[davesparks]

Has RCD protection as well.
Most answers confirming what I thought with it being ok but have heard mixed opinions on this before.

What is YOUR opinion? Or do you just follow the herd?