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Old 10th Mar 2018, 4:06 pm   #121
Phil G4SPZ
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Default Re: Don't try this at home

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Originally Posted by merlinmaxwell View Post
Could they just stick "PAT Failed" red stickers on stuff and then sell it?
Of course, but not for so much money...
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Old 10th Mar 2018, 5:27 pm   #122
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Default Re: Don't try this at home

We probably shouldn't go on too much about PAT testing, but I'd just like to say that the tester should definitely NOT power up the equipment to see if it works, as he/she is NOT qualified to know how a particular piece of equipment should operate and could cause serious, expensive or even dangerous damage by doing so. Where I used to work we had some very specialised equipment that would have been seriously damaged if power were to be applied, even for a few seconds, without certain other ancillaries being connected and turned on first. In the very early days of PAT testing at our place, there was some very costly damage caused by testers who weren't sticking to the instructions and were applying power to expensive equipment without having the remotest idea of the damage they were causing. As PAT testers became better trained, this problem was no longer encountered. Applying power to an item by an uninitiated person who has no idea of the operation or purpose of the item could in the worst case cause an explosion and and possible injury. In those early days it did take some considerable effort to get it into the heads of these people that they must NOT put mains power onto ANY piece of equipment unless they fully understood its operation, and if in any doubt, to ask a qualified member of the staff.

Among many items of equipment, other than specialised computer controlled equipment that could be damaged, were such things as high vacuum diffusion pumps that needed a cooling water supply attaching, turning on and running, as applying mains for even the shortest time would have spelled disaster. Being aware of this, I myself had to keep watch and step in to stop the vacuum kit being destroyed on several occasions during those early days of PAT testing, and that wasn't the only kit I had to keep a watchful eye on while these testers were on the site.

So definitely NO applying mains and operating equipment by P.A.Testers.
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Old 10th Mar 2018, 5:58 pm   #123
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Default Re: Don't try this at home

I was really confining my comments to domestic portable electrical appliances likely to be found in the home, office or charity shop (desk fans, lamps, hairdryers, radios etc) in the context of the preceding discussion in this thread, and not specialist industrial equipment.
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Old 10th Mar 2018, 6:24 pm   #124
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Default Re: Don't try this at home

It's a good point you make, Phil. There's still the possibility of damage to even certain domestic items that could be damaged. However there's then the very good point that could be argued that if the item is likely to be damaged by a tester applying mains power, then that would be a very good reason for the item not to go on sale to the general public in a charity shop.

I think the bottom line is that there doesn't seem to be any proper regulations for testing and really the whole system needs to be brought into some sort of line.

The cutting off of mains leads on stuff in auction houses I personally think is a bit daft. If you were to do a proper risk assessment on the most likely outcome of chopping off a mains lead, you'd probably find that it would be more likely for an unqualified person to try and reattach another length of cable by twisting the wires together and insulating with sellotape with disastrous results, than to leave as is and just have a small 'pop' and a blown fuse if the old piece of kit is just plugged in by the buyer.

I think there's a risk we may get told off for talking about PAT testing too much, but I also think it's a very serious subject that impacts on what we do in our hobby and should be openly debated.
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Old 10th Mar 2018, 6:39 pm   #125
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Default Re: Don't try this at home

Even though some non-technical person is more at risk from their own bodging of a cut-off mains lead than from the piece of untested equipment, the responsibility will then rest with him.

What was started with the laudable aim of preventing death or injury has transformed into an exercise of shifting responsibility, even at the cost of increasing the risk of death or injury.

It's all about blame now, not about danger.

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Old 10th Mar 2018, 9:16 pm   #126
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Default Re: Don't try this at home

The additional danger often forgotten with the amputation of mains plugs is the plug itself if not disposed of securely. Once chopped, it is dangerous if plugged in, given that the inner wires are exposed at the point of amputation.

One item I bought at an auction (not the same auction as I referred to above) had had this operation performed, but the severed plug was included with the item. Had I been daft enough to connect said plug to the mains and electrocuted myself, they could probably have been deemed liable. The really galling thing was that the wires I had to remove from the plug to reconnect the item had crimped-on ends, whereas I ended up with just the normal bare wires screwed into the terminals.
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Old 10th Mar 2018, 9:31 pm   #127
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Default Re: Don't try this at home

Many years ago there was a magazine called "old Moores almanac" it had loads of things in to build and use around the home. My grandfather was a big fan of this and made lots of the things in it.the bedwarmer consisted of a 150 watt bulb in an oven glove, he had no 150W bulbs, or oven gloves, not a problem, they had 200W bulbs and asbestos mitts at work. He "borrowed" the necessary items, et voila, one bed warmer!
The second one was an electric alarm clock, a bare cable, straight from the mains was attached to the little hand of a clock, the other cable was left sticking out from the face at the wake-up time, this operated a 240v buzzer and woke you up, assuming of course that you didn't make accidental contact with the live wires and electrocute yourself.
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Old 10th Mar 2018, 9:47 pm   #128
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I suppose they could! The final stage of the PAT is to switch on and see it works, but of course you couldn't guarantee a computer, for example, was in full working order! Hence the disclaimer.
My tester runs you through the tests you should do FIRST - i.e. 'Is plug OK? Is lead OK? Is fuse OK? Is case OK?'. Only when you've said yes to each of these can you start the powered test. In my experience most failures occur before the test proper.
Without doing a "run test" then a PAT is a waste of time, I have a couple of testers and they both do a run test that checks the amount of current being drawn. These are expensive pieces of kit, unfortunately, most people just use the cheapo basic ones, these just do IR and earth continuity.But as others have said, the big one is the visual inspection, I was nearly electrocuted in a nursery, by a piece of kit that had been tested recently, it transpired that the owner, wanting to save money, had done a one day PAT course, and bought a cheap tester. He had tested all the kit in the nursery, however, he'd not done a visual inspection, if he had, he may have noticed the loose cable hanging out of the plug on the hoover that his machine had said was ok!
Personally, if it was up to me, I'd only permit testing to be done by a qualified electrician (I have over 30 years experience), most of these "so-called" test engineers, have little if any idea of what they should be looking for, and don't get me started of flash testing!
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Old 10th Mar 2018, 10:46 pm   #129
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Default Re: Don't try this at home

I would never consider a piece of equipment to be adequately tested, unless an internal visual inspection has been made.
I've seen too many bodges on SMPSs in metal cased class II equipment that would probably register a "Pass" if tested in the normal way.
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Old 10th Mar 2018, 11:03 pm   #130
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Default Re: Don't try this at home

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He had tested all the kit... however, he'd not done a visual inspection, if he had, he may have noticed the loose cable hanging out of the plug...
Absolutely. I remember some years ago the then IEE published a report on the state of electrical safety of household appliances, which (from memory) concluded that about 70% of all appliances had defective plugs, flexes, incorrect fuses, crossed wires, loose cord grip screws and a whole raft of other problems.

None of these are detected by a PAT unless the visual inspection is undertaken with care and diligence by someone who has been trained to spot the faults, and warned about the dire consequences of lack of diligence!
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Old 11th Mar 2018, 2:58 am   #131
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Default Re: Don't try this at home

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Among many items of equipment, other than specialised computer controlled equipment that could be damaged, were such things as high vacuum diffusion pumps that needed a cooling water supply
Here at home I run an HV system with a small vapour diffusion pump (Edwards EO2 with a mere 400 watt heater). I take strong precautions against me accidentally applying power when its not being cooled and not under vacuum. The very thought of someone just applying power to it doesn't bare thinking about. I use a silicon oil, so no explosion risk but it would make a horrible mess of the insides of the diff pump. Were it to be on an ester or other synthetic oil there could be serious consequences if it were not under vacuum at the time.

When I was still working at the Uni as an IT professional we didn't let them anywhere near our servers. Luckily our server room was deemed a "secure area" so we had some control over them.
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Old 11th Mar 2018, 5:43 am   #132
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Default Re: Don't try this at home

Interesting, what do you evacuate?
Ever thought of re-gunning CRTs? You could become VeRy popular!
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Old 11th Mar 2018, 3:19 pm   #133
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Default Re: Don't try this at home

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It's all about blame now, not about danger.

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Old 11th Mar 2018, 4:29 pm   #134
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Here at home I run an HV system with a small vapour diffusion pump (Edwards EO2 with a mere 400 watt heater). I take strong precautions against me accidentally applying power when its not being cooled and not under vacuum. The very thought of someone just applying power to it doesn't bare thinking about. I use a silicon oil, so no explosion risk but it would make a horrible mess of the insides of the diff pump. Were it to be on an ester or other synthetic oil there could be serious consequences if it were not under vacuum at the time.

When I was still working at the Uni as an IT professional we didn't let them anywhere near our servers. Luckily our server room was deemed a "secure area" so we had some control over them.
That brings back memories... I spent 5 - 6 years running HV lines in Universities, but with Mercury diffusion pumps in those days - it was never good on the nerves, although to be honest, we probably worried more about condensing liquid O2 into the traps. I've seen those go blue a few times...
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Old 11th Mar 2018, 5:07 pm   #135
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Default Re: Don't try this at home

To clarify my earlier post, I was talking about PAT. This implies the item is portable, though many people take it a to mean any item with a mains plug on it. I wouldn't consider an HV system portable!
As with everything, it's a compromise. I'd be happy to dismantle an appliance to check everything was OK with previous repairs and for electrical integrity. However, charging for said services could be a problem...
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Old 12th Mar 2018, 12:09 am   #136
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Interesting, what do you evacuate?
Ever thought of re-gunning CRTs? You could become VeRy popular!
What I evacuate is a 12" diameter 14" high borosilicate glass bell jar. It has electrical feed-throughs so I can power the experiment in the jar. I have made several simple electron guns. These produce a fine beam, a fan of electrons (narrow but about 2cm wide) and very wide beam. The wide beam I've used to show the Maltese Cross, only mine was a simple cross shape. The two photos attached are of a medium beam gun in action and the wide beam with the electron shadow of the cross nice and clear.
The system gets down to 2*10E-05 torr fairly quickly, usually about an hour after diff start.
After that, the system enters the "drying zone". The diff spends all its efforts trying to pull endless layers of adsorbed water off the inside of the system. The pressure goes on going down and when I've done a particularly long experiment I have seen it down to 3*10E-06 torr.

Yes I could pull a good enough vacuum for a CRT, particularly as the ultimate pressure in all valves is achieved by firing a getter. The problems of re-gunning a tube are formidable. Roughly these are:
1 - glass blowing skills
2 - Source of electron guns
3 - Source of barium/aluminium getters
4 - Large enough oven to heat the entire tube for 2 hours while pumping at something like 250C.

4 is needed to drive off all these layers of adsorbed water that limit my ultimate vacuum.

Oh and I forgot, high power external induction heaters are needed to heat all the metal
parts to dry the during pumping and after that to flash the getters.
Love to try though!
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Old 12th Mar 2018, 12:35 am   #137
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Default Re: Don't try this at home

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To clarify my earlier post, I was talking about PAT. This implies the item is portable, though many people take it a to mean any item with a mains plug on it. I wouldn't consider an HV system portable!
Here in Oz, anything with a plug on it must be tested, regardless of size and 'portability'.

The acronym PAT (Portable Appliance Tester) refers to the equipment used for testing rather than what is being tested Downunder.
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Old 12th Mar 2018, 3:17 am   #138
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Default Re: Don't try this at home

Nice bell, glad to see the shield.
I have done some glass work years ago, re-necking etc.
Finding someone to assemble guns is the sticking point, oh, and a glass lathe!
As there is now nobody re-gunning there is an opening if the volumes could be assured.
As I said, you would be popular world wide.
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Old 12th Mar 2018, 3:28 pm   #139
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Good you have some glasswork experience Sam. Yes a glass lathe would be required. The guns I make for my experiments might be good enough for rebuilt tubes. The ones in early picture tubes were just about as primitive as mine!

Rather than making new guns, it is also possible to replace the cathode in an existing gun. Again very early tubes had less complex (and larger) guns making them potentially easier.

Early CRTs used glass that is significantly different from modern glass. That means making a "graded" adapter between old and new glass to make the change of expansion coefficient low enough it won't crack.

I'd love to have a go at either re-evacuating an old valve or making my own and sealing it off and gettering it. I've looked for getter material but can't find a source. The best and most often used in valves seems to be barium powder. It was held in the getter "boat" by sintering I think. Barium isn't stable in air, it reacts with oxygen, so the getter powder was an alloy of barium and aluminium. Probably only a small amount of aluminium was added, but I don;t know the exact ratio.

I have read that magnesium is a reasonable getter and I've obtained some magnesium powder to try gettring with that.

Well noticed about my safety shield. It, and the nature of the glass in the bell jar, do make my photos less distinct than I'd have liked. My HV system really is a case of "don't try this at home" kiddies! A simple sum will get you an idea of the pressure on my bell jar. It comes out at about TWO TONS, say a big family car, sitting on it. The acrylic shield was bought from a vacuum system supplier and cost BIG money. I had to have it for my own safety and of course I wanted to be able to see into the chamber. That really was essential.

When I first roughed the bell jar down I hadn't bought the shield yet. I started the roughing pump from outside the room. I reasoned if it was going to implode it would go as the pressure went down. Once at the limiting pressure of the roughing pump, if it took the pressure for 10 minutes it would "probably" be OK. I entered the room wearing a heavy motorcycle jacket and helmet with the visor down!

If anyone wants the address of the vacuum specialist supplier please send me a personal message. They are helpful and will supply to individuals.
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Old 12th Mar 2018, 7:14 pm   #140
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Default Re: Don't try this at home

Circa 1980 there was an article in "Wireless World" about the refurbishment of two CRTs for one of the Science Museum's pre-War TVs (a model where you viewed via a mirror in the lid). It was mentioned that the tubes were of Pyrex glass, and they had had to get someone who was about to retire (or already had) to do the guns as he was one of the last people experienced in working with the particular type of glass that they were made with.
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