CPC break - share another?

WP#notaspark · Sep 28, 2023

Hi! DIYer here. I have an MCB running to a cooker isolator switch. The RCD connected to this set of MCBs is tripping and isolating the cooker MCB is the only way to stop it. So, clearly a fault on this line.
I‘ve disconnected the cooker and the same fault exists. I’ve tested the cable for continuity and resistance. There’s a break / fault in the CPC line from distribution board to isolator switch. I’ve isolated the cable at several accessible points and discovered that the break is on a section that runs under a concrete floor.
Running a new cable is very tricky in my situation but may be the only answer. But….. as I have good resistance and continuity on the live and neutral lines, could I use the CPC from a ring close to the cooker isolator?
As stated, I’m a DIYer so be polite if this is a stupid idea.
Thanks in advance.

mainline · Sep 28, 2023

WP#notaspark said:
Hi! DIYer here. I have an MCB running to a cooker isolator switch. The RCD connected to this set of MCBs is tripping and isolating the cooker MCB is the only way to stop it. So, clearly a fault on this line.
I‘ve disconnected the cooker and the same fault exists. I’ve tested the cable for continuity and resistance. There’s a break / fault in the CPC line from distribution board to isolator switch. I’ve isolated the cable at several accessible points and discovered that the break is on a section that runs under a concrete floor.
Running a new cable is very tricky in my situation but may be the only answer. But….. as I have good resistance and continuity on the live and neutral lines, could I use the CPC from a ring close to the cooker isolator?
As stated, I’m a DIYer so be polite if this is a stupid idea.
Thanks in advance.

Running a new cable is the only proper way, your cable is damaged it needs replacing.

James · Sep 28, 2023

The most likely thing that has happened is the cable has been damaged during installation or possibly due to some slight ground movement.

I suspect that there has been a short from live to cpc that has blown apart the cpc. the live conductor is still intact but may well have lost a lot of its cross section.

An insulation test is likely to show a poor reading between live and cpc and that would confirm my hypothesis.

I know this is not the answer you were hoping for but a new cable is the correct thing to do here.
if you are going to channel the floor to lay it in, here are some suggestions.

check for any other services before cutting. i.e. underfloor heating, other cables or pipes etc.
always burry cables under concrete in a duct (pipe) so that they can be replaced in future without digging the floor up.

if you grind a straight line through the concrete and another parallel at about 8 inches apart then cross grind every meter, you can normally lift out the original floor in sections.
dig a small channel in the sand below (2 inch x 2 inch) lay in a 2 inch duct and then back fill with the sand removed until it is level as it used to be.
the original concrete slabs, removed earlier can be put back in and bonded by using an epoxy filler in the gaps created by grinding.
this is only ok for light foot travel, not for commercial spaces or industrial.

WP#notaspark · Oct 9, 2023

James said:
The most likely thing that has happened is the cable has been damaged during installation or possibly due to some slight ground movement.

I suspect that there has been a short from live to cpc that has blown apart the cpc. the live conductor is still intact but may well have lost a lot of its cross section.

An insulation test is likely to show a poor reading between live and cpc and that would confirm my hypothesis.

I know this is not the answer you were hoping for but a new cable is the correct thing to do here.
if you are going to channel the floor to lay it in, here are some suggestions.

check for any other services before cutting. i.e. underfloor heating, other cables or pipes etc.
always burry cables under concrete in a duct (pipe) so that they can be replaced in future without digging the floor up.

if you grind a straight line through the concrete and another parallel at about 8 inches apart then cross grind every meter, you can normally lift out the original floor in sections.
dig a small channel in the sand below (2 inch x 2 inch) lay in a 2 inch duct and then back fill with the sand removed until it is level as it used to be.
the original concrete slabs, removed earlier can be put back in and bonded by using an epoxy filler in the gaps created by grinding.
this is only ok for light foot travel, not for commercial spaces or industrial.

New cable run and all working. Thanks for the advice.

WP#notaspark · Oct 9, 2023

James said:
The most likely thing that has happened is the cable has been damaged during installation or possibly due to some slight ground movement.

I suspect that there has been a short from live to cpc that has blown apart the cpc. the live conductor is still intact but may well have lost a lot of its cross section.

An insulation test is likely to show a poor reading between live and cpc and that would confirm my hypothesis.

I know this is not the answer you were hoping for but a new cable is the correct thing to do here.
if you are going to channel the floor to lay it in, here are some suggestions.

check for any other services before cutting. i.e. underfloor heating, other cables or pipes etc.
always burry cables under concrete in a duct (pipe) so that they can be replaced in future without digging the floor up.

if you grind a straight line through the concrete and another parallel at about 8 inches apart then cross grind every meter, you can normally lift out the original floor in sections.
dig a small channel in the sand below (2 inch x 2 inch) lay in a 2 inch duct and then back fill with the sand removed until it is level as it used to be.
the original concrete slabs, removed earlier can be put back in and bonded by using an epoxy filler in the gaps created by grinding.
this is only ok for light foot travel, not for commercial spaces or industrial.