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.

 

[Bailey326]

Depends how I feel on the day.

 

[TJ Anderson]

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?

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.
[automerge]1592523436[/automerge]

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.

 

[Ian1981]

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.
[automerge]1592523436[/automerge]


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?

 

[Let me think]

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.
[automerge]1592523436[/automerge]


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.

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.