Power cut

[The Medway Handyman]

So, there I was watching an episode of Frost I hadn't seen before - and suddenly the power went.

Fortunately I'd brought two headlight torches in from the van to change the batteries and they were still on the hall table. Popped outside to find every third house out. I assume this was because one phase of the mains cable had failed? (We live in a cul-de-sac with a sub station at the end of the road).

Spent the rest of the evening reading a book by the light of a headlight torch - another use for them. I shall keep one indoors now.

Anywho, this though of phases reminded me of a company called Hydromist who made carpet cleaning machines years ago. They had a machine which had two mains cables, one running the pump & vacuum and the other running a 3kw immersion heater in the water tank.

They sold a load of them to a Govmint Dept for in house cleaning, think it was the DHSS in them days. Someone latched onto the fact that in commercial buildings it would be possible to plug the two cables into two sockets wired off different phases and what with water being present, this would present a major safety hazard.

Cost them a fortune, IIRC they had to take all of the machines back & remove the heaters.

But I've always wondered - how dangerous would this have been? What could have happened?

[Tim S]

The Medway Handyman coughed up some electrons that declared:

Yes it does. Probably a cable fault and/or a fuse in a link box blew (those are often 300A or 500A fuses). The fact you're in a cul-de-sac meant your road is likely to be wired as a "spur" whereas other roads are more of a network with feeds from both ends.

Well, there *could* have been 415V between the phases, so a) the machine's insulation might not have been designed with this in mind (slight oversight if so), or b) there was concern over 415V between two bits of flex in close proximity to a possibly damp user. Cheers

Tim

[The Medway Handyman]

Interesting. I've always wondered about how roads were wired up. I live in Bramley Rise on this map

that give a clue as to how its wired? The sub station is just above the 'R' in Rise.

Ah. That makes sense, the cables and the operator would almost certainly be on a damp carpet.

[Jason]

There *would* have been 415V between the phases, surely?

[Tim S]

Jason coughed up some electrons that declared:

Well, if they *were* plugged into different phases :)

Otherwise there would be 0 volts (more or less) between the phase conductors in the two flexes.

OK - being a pedant ;->

[Rod]

Is this really any different to plugging in a scrubber and a wet suction machine into different phases? Quite likely for the operator to touch both machines at the same time. (I have certainly operated a scrubber with one hand and at least moved a suction thingy with the other. Maybe even pulled it across behind me - but cannot now be certain. 'Twas a long time ago.)

[meow2222]

and unless I'm mistaken it would make no difference. The voltage from earth of any circuit point is the same regardless of whether one plugs into single phase or different phases.

NT

[Tim S]

[e-mail] coughed up some electrons that declared:

Quite. I think Dave mentioned "government" - so there's probably a safety officer who needs to earn his coffee...

[The Medway Handyman]

Good point that.

[Lobster]

I have one which lives on bedside table, for that very purpose. SWMBO is such a light sleeper that if I want to read in bed it's the only solution. And even then I can only have it on the lowest setting, and have to keep my head turned away from her, and I'm not allowed to read any 'large' books as too much light gets reflected back from the white pages etc etc etc.

Have to say I keep fairly quiet about it IRL (except when broadcasting to the world behind an anonymous moniker).

Q. "What do you wear in bed?"

Marilyn Monroe: "Chanel No. 5" Lobster: "An Aldi headtorch"

[Andy Champ]

I've seen exactly this - some properties out, and others not - and it was the same thing. A single phase fault.

A little unfortunate that this happened while I was on a 2 hour car journey to visit my son and fix his washing machine. And while his flatmate was in bed with flu.

I hate leaving untested fixes, but this time I did. Even knowing I might be in for another 4 hour round trip if I hadn't fixed it, I have a job to go to!

Andy

[Harry Bloomfield]

The Medway Handyman explained on 25/01/2009 :

Every third house would mean one phase lost.

Basically there was the possibility of providing the machine with two phases with 415v between them. If the machine was wired correctly, then it would have been of no consequence to the machine - but should the operator manage through some fault to touch both at the same time he would get a much more serious 415v shock. Add in to that a perhaps wet operator and it becomes even more serious.

[Harry Bloomfield]

Tim S presented the following explanation :

Would have been!

The insulation is the same irrespective of single or three phase. With single phase there is 240v between phase and neutral or earth. With three phase the voltage remains the same to neutral or earth, but each phase conductor has its own layer of insulation so two insulation layers separating them and each specified to much more than 240volts x2 = 480v.

[Tim S]

Harry Bloomfield coughed up some electrons that declared:

But was it type tested under such conditions?

I'm not arguing with you Harry - but you have to put a different brain in when trying to see things from a H&S nazi POV (!) Sometimes those folk invent problems where there are none, but if you don't have a bit of paper to disprove them they can't cope with common sense.

[<me9]

The /one/ incident I know about when 415 (now 400) was a problem was one I heard about from an ex colleague (before my time, he then owned the garage where my car was serviced) many years ago. Where we worked had an open switchboard (still in use in the early 70s) with knife switches for the 415v. Below this switchboard was a rubber mat, about an inch thick. We kept our distance in the 1970s. In his day a routine check was to feel the blades of the switches to check for any overheating. Fine whilst standing on the rubber mat.......... until someone in a hurry decided to check two at once.

It thereafter ceased to be a routine check, for some strange reason.

[Andy Champ]

Not 240 x 2. The phases are not opposite, they are 120 degrees out of phase, which is where the 415v comes from.

Andy

[Tim S]

Harry Bloomfield coughed up some electrons that declared:

Just spotted this.

Harry: It's the square root of 3 for a system of sine waves, 120 degrees phase angle between sines, or about 1.7 factor.

2x would be for 180 degrees (2 phase[1]) [1] Although when London had 2-phase supplies, 90 degrees was common. They even had transformers to interconnect 2 phase and 3 phase ("Scott" and "LeBlanc" transformers IIRC).

Cheers

Tim

[Clot]

The creeping disabilities of old age - Glaucoma and cataracts - and a similar SWMBO'd due to being a light sleeper made me carefully mention one day that I thought I'd put a slightly brighter lamp in the bedside fitting - my side not in the middle or anything like that - *******. I managed to live; luckily her handbag was not to hand. I should have got on with it and then seen whether she found out!

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