Losing neutral in the utility system

[sophiecentaur]

Just for info, in the early 1980s I worked in an RF screened room with hefty filtering on the 240V supply. While trying to trace the source of some RF interference, I tested the mains supply in the room and was able to draw about 7 amps at 14 volts between neutral and the earthed walls of the room.

I wonder who paid for the energy.

 

[sophiecentaur]

Just for info, in the early 1980s I worked in an RF screened room with hefty filtering on the 240V supply. While trying to trace the source of some RF interference, I tested the mains supply in the room and was able to draw about 7 amps at 14 volts between neutral and the earthed walls of the room.

I imagine that the 14V Neutral voltage was due to the internal resistance of the 'hefty filtering' - i.e. the current supplied to the equipment inside the room was enough to cause a 14V drop through the N leg of the filter. Was there 14V on the N wire, upstream of the Room?

 

[Guineafowl]

I realise this argument is months old now, but I must point out a few (hopefully interesting) things...

5mA is a personnel protection standard widely recognized in NA and elsewhere, from what I'm seeing your equal is 30mA and we are talking at twice the voltage, meaning it can get through twice the path resistance. Ventricular fibulation occurs between 50-100mA and this is why in NA the 30mA standard is considered for equipment protection only and is insufficient to protect people from electric shock. You call me xenophobic, I call you reckless and where does it get us? But go ahead and argue how a smaller tolerance is somehow less safe.

Now, of course a lower fault current limit is safer. But there’s a reason for the 5/30 difference, and I think I see it...

And secondly, do you not realize that scores and scores of 5mA GFCI devices are in use every day, even on things like vending machines? What, do you think someone has to go around and reset GFCIs every week or two so that they can ensure their sodas are still selling?

I assume this is about a dedicated GFCI for each machine? If so, there’s your difference. UK circuits are usually protected in two groups fed by an RCD each. We’re probably going to move to RCBOs for each circuit one day, but that’s the way it is now. In an ideal world, those RCDs should be 5 mA, but this is reality, and earth leakages, even from new equipment, add up, and at 240V will be twice that seen in the US. Indeed, with a peak voltage of 330V or so, there may be dielectric breakdown twice per cycle that wouldn’t happen at 120V at all. You say 30 mA is a dangerous threshold, and it may be, but the real-world shock current will be the difference between the ‘normal’ leakage and the threshold.

A 5 mA RCD protecting a ring main, an immersion heater, lights, a shower... I bet you any money this will ‘nuisance trip’. This is extremely annoying. And if you squeeze safety standards too hard, what happens? At least one person out of the 70 million here will bypass it.

A final point: We’ve had the dual 30 mA RCD system here for some time now, so we’ve ‘sucking and seeing’ for a while. If a single death occurred despite the RCD working as it should, there’d be an inquest and a call for revised standards. I challenge you to find such an incident, because I can’t. The 30 mA threshold is there not because we think a shock below this is perfectly healthy, but to achieve a balance between safety and usability. As it stands, nuisance tripping is rare, and fatal shocks are rare.

 

[krater]

Now, of course a lower fault current limit is safer. But there’s a reason for the 5/30 difference, and I think I see it...earth leakages, even from new equipment, add up, and at 240V will be twice that seen in the US. Indeed, with a peak voltage of 330V or so, there may be dielectric breakdown twice per cycle that wouldn’t happen at 120V at all...

Yes I suppose this was somewhat addressed in above discussion when I believe I mentioned that 120V to ground is a maximum exposure safety standard for operators per ANSI/OSHA standards for many industries. Many, many control and operation functions that used to be carried out in potentially hazardous environments through 120V circuits are now using 24VDC or less as a control voltage, with a power limited source to further increase safety.

I have seen three or four 120V GFCI devices used for temporary power during building construction, many of which were split initally at what we call a duplex receptacle, many plugins of which had either a long extension cord or a three-way pigtail splitter splitter, or both or even other combinations thereof plugged into them, which ran multiple battery chargers/lights/electric hand tools fed from cumulatively hundreds of feet of cord. And with proper attention paid to the installation and the conditions of the cords or tools connected to them, there are surprisingly few problems with spurious tripping of the protective devices. Nearly every time it happens it's either related to a damaged cord that when removed solves the problem, or some more complicated situation involving something like a large motor that can often be resolved by re-arranging a few things.

 

[Guineafowl]

Yes I suppose this was somewhat addressed in above discussion when I believe I mentioned that 120V to ground is a maximum exposure safety standard for operators per ANSI/OSHA standards for many industries. Many, many control and operation functions that used to be carried out in potentially hazardous environments through 120V circuits are now using 24VDC or less as a control voltage, with a power limited source to further increase safety.

I have seen three or four 120V GFCI devices used for temporary power during building construction, many of which were split initally at what we call a duplex receptacle, many plugins of which had either a long extension cord or a three-way pigtail splitter splitter, or both or even other combinations thereof plugged into them, which ran multiple battery chargers/lights/electric hand tools fed from cumulatively hundreds of feet of cord. And with proper attention paid to the installation and the conditions of the cords or tools connected to them, there are surprisingly few problems with spurious tripping of the protective devices. Nearly every time it happens it's either related to a damaged cord that when removed solves the problem, or some more complicated situation involving something like a large motor that can often be resolved by re-arranging a few things.

Ah. On building sites, a site transformer is often used, stepping the 240 V down to 110V with a centre tapped PEN. This means an operator is only exposed to the potential between one tap and the PEN, i.e. 55V.

My mate’s oven was recently nuisance tripping the RCD. As it cooled, a relay would click and sometimes, not always, the RCD would go. I measured the peak fault current using an adaptor to allow a current clamp around the live and neutral combined. It never went above 30 mA, more like 9 or 10. I guess here, the whole house earth leakage (an old supply, with one RCD protecting everything - crap) was topping up the oven’s contribution. Either that or the old RCD’s threshold had drifted. Anyway, the oven was brand new so it went back under warranty and there have been no problems since.

If your earth loop impedance is low enough, you may have some circuits not under RCD protection - this is common for ovens. Otherwise, it’s RCDs everywhere here.

 

[Guineafowl]

proper attention paid to the installation and the conditions of the cords or tools connected to them

Ha ha ha (hollow laugh).

You haven’t seen some of the things I’ve seen, man. You weren’t there, man.

1. Plug fuses bridged with foil, wire, or a segment of six-inch nail. (“It keeps blowing, so I made another one”).
2. Scuffed cables with live exposed. (“Watch that cable - it’s a bit dodgy”).
3. Twisted-together wires, wrapped with tape and lying on a wet lawn.
4. Above half-submerged in a small puddle.
5. Damaged plugs where the back comes away when you try to pull it out, exposing all connections.
6. Metal light fittings that “keep tripping the RCD”, so they remove the earth wire, and allow the casing to float happliy.
7. A dishwasher I bought from a friend - “turn it off before you open it - the metalwork gives you a tingle” - leaky EMI cap across input. Actually it was blown almost to bits.

Etc.

There’s always a danger that too safe a system results in blunted danger perception.

 

[krater]

Ha ha ha (hollow laugh).

You haven’t seen some of the things I’ve seen, man. You weren’t there, man.

Oh come on now. All these sound pretty tame and don't involve conductors the size of your thumb or noises similar to a freight train happeningn inside equipment. Those of us who are intimate with the animal are by and large very aware of the risk. The remainder of the populace is almost totally oblivious to the hazard. Herein lies the approach of applying safety standards.

 

[Guineafowl]

Oh come on now. All these sound pretty tame and don't involve conductors the size of your thumb or noises similar to a freight train happeningn inside equipment. Those of us who are intimate with the animal are by and large very aware of the risk. The remainder of the populace is almost totally oblivious to the hazard. Herein lies the approach of applying safety standards.

I’d say the vast majority of people, at least in the UK, are indeed oblivious to the ins and outs of electricity. As such, they view it as a kind of dangerous black magic, and won’t dare to fiddle. Hence the many call-outs electricians get to simply reset someone’s MCB.

Farmers.

By and large, the salt of the earth. Problems:

1. They are used to doing things for themselves.
2. They need things done now, so they can get on.
3. They have piles of junk that can be used to cobble together solutions to problems.
4. They are used to working with clapped-out gear.
5. They are practical people with tools and a little knowledge.

You’ll always find the worst, excremental electrical setups on farms. If that’s not a testament to the 30mA threshold, I don’t know what is.