Fluke T130 on loose wires

[Deleted member 267285]

The SAFEST answer is isolate the whole thing as the MCB only breaks the line [live] conductor and no amount of 'testing for dead' will assure perfect safety once terminals are undone. Borrowed or Shared neutrals used to be very commonplace, this is where 2 MCB's use the same neutral for all or part of the circuit and particularly for 2 way switched lights between floors.

In my current house I made significant changes to the lighting circuits over a 20 year period and didn't know I had 2 lights using the wrong neutral until squirrels chewed a cable in the loft. Having turned off the upstairs lights MCB I disconnected the damaged cable and promptly got a belt via the landing light which was supplied by the downstairs lights MCB but on the unstairs neutral.

These days with RCBO's there is a hugely reduced risk of finding such issues but faults do occur and when someone asks the question, I'll assume they have less experience than me to cope with the unexpected and for their [your] safety I'll give the same answer. This is definitely NOT an insult, this is me offering safety advice.

I personally wouldn’t consider it being an insult, as I like to work safely. Fantastic advice, especially about the shared/borrowed neutrals, so thank you. Obviously a seasoned professional. It’s also nice to have advice where you’re not being talked to like you’re an imbecile.

 

[SUNRAY]

I personally wouldn’t consider it being an insult, as I like to work safely. Fantastic advice, especially about the shared/borrowed neutrals, so thank you. Obviously a seasoned professional.

I appreciate that saying "I know more than you" can seem to be an insult and some would take it that way. Let's just say I remember helping Daddy do electrical work in a house we moved out of in 1961. Sometimes the 'unexpected' comes from very unexpected places, like between 2 different earth systems.

 

[JohnW2]

Would this mean that every time I am working on any circuit (for any duration), I need to put the whole house out of action? Is it safe to isolate that individual circuit (once appropriately tested) and then lock the MCB before working on this?

I suppose it depends upon how ultra-cautious (or 'risk-averse') you are. In a domestic environment, if the MCB is locked off and proper 'testing for dead' has confirmed that the circuit is 'dead', then any residual risk is incredibly small - many/most would probably say 'negligible'.

Any theoretical risks necessarily relate to cross-connections of one sort of another between different circuits and, not only that but, if the circuit initially satisfactorily 'tests dead' then the effects of such 'cross-connections' would have to be 'intermittent' (or, as below, only present if something was cut/disconnected) if they were ever going to present a risk.

Probably the greatest potential risk is that of a neutral being shared between circuits (a 'borrowed neutral') - which, if cut/disconnected (on a 'dead' circuit) can create a risk under some circumstances. Since that is something which, if it exists, is almost always (in domestic installations) between two lighting circuits, then one might regard it as wise to lock off the MCBs of all lighting circuits when working on them. However, if you have two lighting circuits protected by different RCDs (or RCBOs), then a 'borrowed neutral' cannot exist without one or both of the devices tripping - hence giving reassurance.

I would imagine that very few are as ultra-cautious as SUNRAY suggested.

Kind Regards, John

 

[Jupiter01]

is almost always (in domestic installations) between two lighting circuits

Appreciate the comments on lighting circuits. On domestic systems, how common is the "neutral risk" with RFC's?

 

[JohnW2]

Appreciate the comments on lighting circuits. On domestic systems, how common is the "neutral risk" with RFC's?

Incredibly rare, I would imagine - it could only really happen if some pretty crazy 'bodging' had been done. It's really primarily because of two-way switching of lights (particularly when some lights are on different floors from a corresponding switch - as with halls/landings) that 'shared neutrals' arise.

As I hope I've explained well enough, the problem with 'borrowed neutrals' is that no amount of 'testing for dead' will reveal any problem so long as the circuits are 'intact' - it's only if/when one cuts or disconnects a neutral that a risk can arise.

Kind Regards, John

 

[RF Lighting]

I used to see borrowed neutrals from the socket ring most commonly where often but not exclusively wall lights had been added. The live was taken from the switch and the neutral from a nearby socket.

It’s rare to see these days especially since the advent of RCDs but still not unknown.

As John has said that testing often does not show a borrowed neutral but it can still give you a nasty shock. The only way to definitely avoid this it to power down the whole installation.

This isn’t always possible once you are out of the domestic environment and I’m in the habit of testing every neutral every time I break a connection but even with this and many years of training and experience I have still received electric shocks in my career

 

[JohnW2]

I used to see borrowed neutrals from the socket ring most commonly where often but not exclusively wall lights had been added. The live was taken from the switch and the neutral from a nearby socket.

I hadn't thought of that one - but I suppose one can 'understand' (even if not 'excuse') how/why that might have happened.

As we've both said, any sort of (persisting) 'borrowed' neutrals has become much rarer with the advent of RCDs - but one still has to be careful/cautious with dual-RCD boards, since I've seen at least some in which two (or more) lighting circuits are protected by the same RCD - which is inevitable if there are more than two lighting circuits (and not unknown even when there are only two!).

This isn’t always possible once you are out of the domestic environment and I’m in the habit of testing every neutral every time I break a connection but even with this and many years of training and experience I have still received electric shocks in my career

These things happen. I've been trying to think of a safe way of 'testing for a borrowed neutral' before doing anything to any of the circuits. As a thought .... I suppose that one could put a clamp meter on the neutrals of each circuit protected by the same RCD (or all circuits, if no RCDs) and see if the current changed in response to loads being applied to circuits other than the one being measured ... or am I missing some flaw in that approach?

Kind Regards, John

 

[SUNRAY]

My safety precautions expanded expotentially when I started doing controls work where multiple sources of power can come together in an enclosure. For example a device, let's say boiler as the same thing applies to domestic boilers, may have 2 terminals for the 'enable', that'll be live and enable, that cabled to the control panel where it ends up on a relay contact [example], there may be 6 boilers. There may be a dozen pump sets wired the same way. Straight away that's 18 live wires entering the control panel and the line of chillers on the roof may add another unknown quantity, I say unknown as many chillers have 4 or 6 or 8 compressors and the same number of fans etc and each may have their own enable or there may be a panel on each chiller. The trouble is in such systems no amout of testing for dead will find the odd bit of 'rogue kit' which starts unexpectedly and introduces another live feed.

OK I'm painting a picture of electrical applications that the vast majority of electricians will never see and therefore totally lacking in experience to work on [and yes I have frequently been called in repair the damage done by electricians in such systems], again this is not an insult, just a comment that qualifications don't come with experience built in.

In any case I frequently find myself working in fully powered control panels [how else does one fault find?] and I believe I've earned enough T-shirts over the years to be competent enough to do so.

However the basics are the same, right through the electrical environment from putting a plug on a cable to thousands of amps of 3phase distribution.
Someone who is not aware of the potential problems is best advised to fully isolate and my experience tells me that if someone is asking the question, that will be my advice. As they gain experience the confidence follows, as does the competence. At his stage I have no idea of Jupiters experience other than the questions in this thread and I hope I've interpreted correctly.

Obviously my advice to the likes of JownW2, secure, RF to name but 3 would be tailored differently and W.1 would be vastly different.

I suppose it depends upon how ultra-cautious (or 'risk-averse') you are. In a domestic environment, if the MCB is locked off and proper 'testing for dead' has confirmed that the circuit is 'dead', then any residual risk is incredibly small - many/most would probably say 'negligible'.

Any theoretical risks necessarily relate to cross-connections of one sort of another between different circuits and, not only that but, if the circuit initially satisfactorily 'tests dead' then the effects of such 'cross-connections' would have to be 'intermittent' (or, as below, only present if something was cut/disconnected) if they were ever going to present a risk.

Probably the greatest potential risk is that of a neutral being shared between circuits (a 'borrowed neutral') - which, if cut/disconnected (on a 'dead' circuit) can create a risk under some circumstances. Since that is something which, if it exists, is almost always (in domestic installations) between two lighting circuits, then one might regard it as wise to lock off the MCBs of all lighting circuits when working on them. However, if you have two lighting circuits protected by different RCDs (or RCBOs), then a 'borrowed neutral' cannot exist without one or both of the devices tripping - hence giving reassurance.

I would imagine that very few are as ultra-cautious as SUNRAY suggested.

Kind Regards, John

 

[aptsys]

I use these:

 

[bernardgreen]

I use these:

That is something I could use, do you have a link to a place from whence I can buy a set.

 

[SUNRAY]

I use these:

View attachment 231073

Oooohhh YEssssssss. I NEED.
So much better than:

 

[aptsys]

They're from this kit: https://uk.rs-online.com/web/p/multimeter-leads/3245975/

But there's many different types available, Pomona and Keysight leads also have these.

Fluke AC175's fit.

 

[aptsys]

Fluke AC175's screw straight on to the little thread on the probes

 

[bernardgreen]

The probe kit that came in a bundle with my Fluke 115 doesn't have small croc clips. These are the smallest