Both RCD's trip at the same time.

[Taylortwocities]

Matey, you are going to have to do some serious sorting out if you try installing a "17th edition" board with dual RCDs and there's a borrowed neutral.

Before installing a new CU you should be doing a PIR and sorting out serious issues before you do the swop.

What would you do, for instance, if you found the measured value of PFC current exceeds breaking capacity of protective device or high values of EFLI on a circuit?
Would you then install the replacement CU and mark the departure on the EIC?
Where's your judgement on this, where's the line that you would draw.
If a borrowed neutral is OK what about no bonding to a gas service? Is a reversed polarity OK?

Just pop in on the cert as a non compliance, I'm sure the client will read it and understand the implications. Good grief

 

[plugwash]

What would you do, for instance, if you found the measured value of PFC current exceeds breaking capacity of protective device or high values of EFLI on a circuit?

Isn't there a rule that basically states in almost all domestic installations the service fuse can be considered to provide back up protection.

 

[Spark123]

Some types of fuse may be used to provide backup protection for a high PFC, high efli is another problem tho.

By installing a new dual RCD CU you arnt making the installation any more dangerous, on the contrary its alot safer than what it was!

Depends wether or not you consider introducing RCD protection common to both circuits more dangerous or not. Personally I'd shy away from doing that.

 

[experience]

Dont get me wrong, i'm not the sort of person who goes to a job and if its dangerous carries on regardless & just notes the dangerous departures on the EIC.

If there are departures that i find which are dangerous which could lead to electrocution, house fire or a protective device inadequate for the circuit its protecting these will have to be rectified.

However the main danger for a borrowed neutral is when it comes to isolation, if you label the CU identifying this fault any future spark will be fully aware and so will isolate the secondary circuit as well. A borrowed neutral is not going to stop the protective device functioning in a fault condition, and so by upgrading the protection device from a semi enclosed fuse to RCD protection with faster disconnection times is IMO making the installation safer than what it was.

What would you do, for instance, if you found the measured value of PFC current exceeds breaking capacity of protective device or high values of EFLI on a circuit? Would you then install the replacement CU and mark the departure on the EIC?

Obviously the new protective devices in the CU wouldnt be adequate for protecting the circuits so you wouldnt leave a CU that didnt protect the user as that would be dangerous! You have set values which must be met so the new CU would have to fall within these values.

OK what about no bonding to a gas service?

Installing the new CU will mean that the bonding is upgraded to both of these so this wouldnt be an issue!

Is a reversed polarity OK?

You'd be installing new tails so this wouldnt be an issue if it was as the main incomer.
If a circuit was incorrecly wired and dangerous you dont energise it and rectify the fault.

Where's your judgement on this, where's the line that you would draw.

The line i draw is if the installation going to be more dangerous from the work i'm carrying out, in the case of a borrowed neutral and fitting a new CU its not any more dangerous. Fitting a metal light with no earth then yes it is.
There are thousands of homes out there with no earthing conductor in their lighting circuits, this is a departure. But aslong as all fittings are not metal it is fine to install a new CU. You will always come across departures, and clients will not want to pay for these. Common sense then prevails and you have to look at the situation and see if the work you are carrying out is making the installation dangerous. Of course in an ideal world all departures would be rectified in every UK home, more money for us sparks and safer installations for the customer - if only this was the case!

So in this case IMO a borrowed neutral wouldnt stop me installing a 17th edition CU.

 

[Spark123]

I'm of the opposite opinion, I wouldn't like to put my name to it knowing there is a shared neutral.

 

[festa]

I work for my local council doing PIR`s on voids and since i started i come across borrowed neutrals at least once a week (on installs 10 years plus) .....NIGHTMARE !!!!

 

[securespark]

I've been at it for 20+ years too. My first experience of RCD's was in 1983.

You would think the guy would at least have an idea of why two RCD's should be tripping together:

Because there is an imbalance between them.

Thinking logically on from this, why is there an imbalance between the two units?

And to my mind, the first things that I think of is (as you say) that the live & neutral are not routed through the same RCD.

OOI, what was running through his mind, aside from the neutral connections at the CU?