High protective conductor currents - Deletion of 543.7.1.204

AJJewsbury said:

+1 for that. In fact the twin terminals have proved very useful on several occasions where I've ended up with 3off or 4off 4mm² c.p.c.s to terminate - for some reason BS 1363's terminal capacity thinking seems to overlook that you often want a fly lead to the back box in addition to the two ring (or radial in and out) and one spur conductors.

Agreed.

However, still don't see many BS EN 60309-2 outlets with two PE terminals.

Simply if the single terminal fails both conductors forming the duplicate cpc may become completely detached thus removing the path for functional earth currents.  With two terminals the failure of one involving one of the duplicate cpc conductors becoming disconnected leaves the other conductor intact in it's separate terminal - thus maintaining a path for functional earth currents.

Regards

Geoff Blackwell 

If a terminal is subject to failure, surely that's the case regardless of the size of cpc connecting to it?

My feeling is that a given terminal may well be less reliable when holding a smaller conductor than a lagrer ones - some of us tend to double over smaller conductors to try and fill the terminal, but that practice doesn't seem to be universal. I'm sure there's a greater fear of overtightnening and damaging smaller conductors, so perhaps a natural tendency for screws not to be quite so tight, and less 'give' in the smaller conductors in any event. I'm sure we've all dropped off a socket front to find the wires popping out of the terminals - perhaps solid rather than stranded nature of the smaller sizes (in T&E) can result in more stress on the connections that makes a contribution too.

    - Andy.

AJJewsbury said:

My feeling is that a given terminal may well be less reliable when holding a smaller conductor than a lagrer ones - some of us tend to double over smaller conductors to try and fill the terminal, but that practice doesn't seem to be universal.

Is there evidence for this? Are we seeing regular problems attributed to this?

AJJewsbury said:

m sure there's a greater fear of overtightnening and damaging smaller conductors, so perhaps a natural tendency for screws not to be quite so tight, and less 'give' in the smaller conductors in any event.

Torque settings and manufacturer's instructions?

AJJewsbury said:

I'm sure we've all dropped off a socket front to find the wires popping out of the terminals - perhaps solid rather than stranded nature of the smaller sizes (in T&E) can result in more stress on the connections that makes a contribution too

Perhaps ... but that could be resolved by requiring stranded (min 7 strands) for conductors used in such circuits as an equally valid installation option?

Don't recall ever seeing Torque requirements in accessory instructions, consumer units yes.

OlympusMons said:

Don't recall ever seeing Torque requirements in accessory instructions, consumer units yes.

Manufacturers are starting to put this information out ... values I've seen vary in the range 1.5  2.5 Nm ... Example here is 2 Nm but other products from the same manufacturer say 2.5 Nm: d2cq9zndfvtoz7.cloudfront.net/CMA032.pdf

Thanks Graham. So torque screwdrivers now required to change a light switch seems the way we are going. At least that fitting has the Blackwell Inheritance.

There are no torque requirements for the mk rapidfix range, they have those sprung connectors, and they have no spare Blackwell Inheritance either.

OlympusMons said:

There are no torque requirements for the mk rapidfix range, they have those sprung connectors, and they have no spare Blackwell Inheritance either.

It's up to the designer / installer what products to use for which purpose.

In any case, we are talking about individual BS 1363-2 socket-outlet plates here, not complying with BS 7288, which, on the vast majority of circuits complying with 411.3.3, would be required to have a maximum protective conductor current of 9 mA, and therefore Blackwell Inheritance not necessary even to BS 7671:2018+A1:2020?

No but they do have two screws, so effectively two connections to the CPC. The real faults are loose screws not broken conductors.

If a circuit has a protective conductor current of 10mA and we lose the conductor, exposed conductive parts do take on a potential but a human being making contact will not have the full 10mA diverted through their body by virtue of their own body impedance.