Question relating to power factors and old welder

Yes I should think that's quite possible, you have to measure (or calculate) the PFC and check it against the curve for the particular MCB to ensure it trips fast enough.

I'll take your word for it Mark as I don't know the welder.

Greg

Yes that's right, though it's totally counter intuitive the worst inrush occurs when switched on near zero crossing which almost inevitably causes core saturation. Greg

It's worth pointing out that a run capacitor connected to a 440V tap has to have nearly double the voltage rating of one connected across the supply which may affect the economics of it.

Greg

Just to clarify, there are graphs of tripping curves for each combination of class (B, C, D) and the current rating, so you check the PFC against the exact one you wish to use.

Greg

My 225A Oxford has never tripped the 40A ClassB through which the supply passes, nor did the 250A set which I used briefly. However my

250A TIG set (transformer/thyristor type) will trip it on switch-on. That *appears* to have some built-in power factor correction, across the 440V terminals of the transformer. I say 'appears' because there's a capacitor symbol across the relevant terminals, with a pair of wires disappearing into the depths of the machine, none of which is shown on the makers' published circuit diagram. Despite that, the power factor quoted on the rating plate is awful.

Cheers Tim

Dutton Dry-Dock Traditional & Modern canal craft repairs Vintage diesel engine service

On or around Sat, 28 Oct 2006 09:32:45 +0100, Mark Rand enlightened us thusly:

I thought that was (one of) the point(s) of using AC, that you could let go?

besides, if you've got a metal machine which isn't earthed, you're asking for trouble anyway - and if a live wire goes astray and hits en earthed metal cabinet then it should trip PDQ.

Of course, if you're running a machine with a high-voltage DC motor by using a sod-off rectifier, then what you say above would apply. But I rather doubt many people are.

'twas actually Greg who spake thussly:-)

Mark Rand RTFM

On or around Sat, 28 Oct 2006 21:17:25 +0100, Mark Rand enlightened us thusly:

yer quite right. I happened to notice it in your followup though.

No, that's an old wives tale, AC will contract your muscles just as well as DC so if you are grasping a handle for example you won't be able to let go.

Another old wives tale is that if something is earthed you can't get a shock from it, in fact you will see typically 2/3 of the mains voltage on an earthed metal part if a phase touches it. It's obvious if you think about it, the metal is earthed through a thinner wire than the phase conductor (usually) but one just as long, the supply voltage at the sub station is applied across a potential divided made up of the two wires so you get a high potential at the metal part. The only place this won't happen is right at the sub-station where the earth and neutral are bonded together and bonded to an earth grid burried under it, but most people are a distace away.

Yes that's the whole point, it should trip quickly so the shock you get doesn't last long enough to do you much harm, but if you don't match the MCB to the prospective short circuit current it may not trip quickly at all.

What I said does apply, it's fundamental stuff that any electrician has to understand.

Greg

Hey! We agree on things :-). Although I would like to see no more than 50-80V on a chassis in an event. That takes specific design action, such as my computer rooms that have distributed Earthing so that the earth impedance is significantly lower than the supply impedance.

I've spent the last couple of weeks sending emails back and fourth with a 'large' 'bluish' Computer manufacturer over whether their use of a particular Burndy multi-pole connector for mains electricity was legal in the UK on the grounds that I felt that even my fingers (let alone an IEC-950 finger test probe) could touch live parts (Electricity at Work Regs). They say that they've installed the kit in multiple paces all over the UK and Europe. I say that if they have, they've installed illegal kit in multiple places. And I don't normally worry too much about safety :-|

Mark Rand RTFM

8-)

Yes you would think there should be some limit in the regs but I can't remember seeing one, the fact that twin and earth has a thinner CPC seems to imply they don't care!.

I find the test pin to be the real trouble with enclosures rather than the test finger, though not sure how that applies to connectors. I assume you're talking about a US company, we're in 'negotiations' with the US supplier of a climatic test chamber for our factory that has no interlocks or even a padlock hasp on the door, you can open it at 100 degrees and 100% humidity and scald people within a 10 foot range!, their attitude is "so what, that's fine in the US". For the country that exported their litigation culture to us they're remarkably lax about safety.

Greg

So its not true then that positive going cycles contract the muscles and negative going ones relax?

Bugger.

Serious question. My mill and lathe are both earthed via the 13amp plug. Both machines are fed from a 60Amp ELCB (All the garage sockets are fed from this).

However I have a 6 foot copper stake burind in the ground on the wrong side of the garage wall - is there any point in tying both machines back to this earthing point? Both machines are within 4metres of it.

TIA

Steve

On or around Sun, 29 Oct 2006 10:37:10 -0000, "Steve W" enlightened us thusly:

I understood it was the presence of a zero in the middle that was the relevant bit.

Mind you, with 3-phase you don't get a zero.

No, not at all, I used to know someone who got himself attached to a live machine and he remembers thinking "if I don't let go soon I'll be dead", fortunately he managed to prise himself off it in time.

In fact AC is the really dangerous one, if you get more than a few mA through your heart it makes the muscles pulse rappidly and even when the shock is removed the heart is stuck in this new rhythm, but the valves can't work that fast so no blood gets punped and you die about 4 minutes later. The only way to stop this is to apply a DC shock, which stops the heart and lets it re-start in a more normal rhythm, that's what a defibrillator does, CPR almost never stops ventircular fibrillation although it can keep someone alive until the equipment arrives.

I have an RCD (the new name for an ELCB) on my garage distribution panel for this reason, although it's technically mandatory on any downstairs ring anyway in case it gets used for garden machinery.

Well it won't hurt, and may lower the loop impedance a bit which is a good thing, it's little different from bonding the supply earth to the water and gas incomers. It's normal to wire the rod to the earth bar in the consumer unit rather than to the machines directly.

What people don't realise is that an earth rod is not necessarily at earth potential!, it can have tens of ohms impedance to earth depending on the ground conditions, and if you think about it 240V/10R is only 24A which will never, ever operate the 32A MCB for the ring. This is why any property not supplied with an earth by the electricity board, thus using their own rod, MUST fit an RCD to the whole installation or they will not have protection against shock by indirect contact, i.e. a wire touching a metal case.

But back to your question, it won't hurt but it may not make a huge improvement as the supply should already be down to a very few ohms, and your RCD should operate faster than anything.

Greg

Snip From personal experiece. If you get a bad handlocked AC shock and can't let go - walk backwards to drag yourself free.

Jim

Useful advice... which I hope I never have to try out 8-) Greg

Interesting - my experience is that I only have a vague recollection of what legs are for at that point!

Steve