Wiki - Appliance repair hazards

[meow2222]

Feedback welcome...

Appliance repair hazards

The main hazards in appliance repairs are summarized. There are always others that have the potential to occur too.

==Common risks== Some risks are common to all mains appliances:

• shock if plugged in

• basic wiring faults

==Microwave== These are one of the highest risk domestic appliances to repair. The EHT supply in particular tends to be fatal if touched, and has killed experienced technicians.

• EHT supply for magnetron

• Parts connected to EHT supply: LT magnetron supply wires, transformer, capacitor, rectifier

• Power supply capacitors sometimes store thousands of volts even when unplugged

• Misalignment or miswiring of interlock switches

• Failed shorting resistor when replacing [[fuse]]

• Microwave leakage after replacing a component

• Microwave leakage from rust holing

• Sharp edges

===Old microwaves=== Old machines from the beginning of the '80s and before can have:

• unsafe interlock system.

• carbon loaded rubber seals prone to coming adrift, permitting leakage

Some 1970s and earlier machines have power controls that switch the EHT directly, making all internal parts of the power control and associated wiring dangerous to touch until the EHT has been fully discharged.

Commercial machines from the 1960s and before are uncommon, but do exist, and in a minority of cases can continue cooking with the door part way open. These are hopelessly unsafe.

===Damaged microwaves=== The mains risks are:

• missing or damaged door grille cooks the user

• warped door

• missing seals on old machines

• damaged door hinges

• door misalignment

==TV== ===CRT TV===

• Power supply capacitors store hundreds of volts even when unplugged. This voltage is distributed around some parts of the circuit board.

• EHT supply, stored by the picture tube when off

• Focus voltage, also stored

• Propped appliance can fall on child or pet

• Picture tube may explode if the unprotected thin rear of the tube is broken

===LCD TV===

• Power supply capacitors store hundreds of volts even when unplugged. This voltage is distributed around some parts of the circuit board.

• High voltage stored in lighting invertor

• Weak thin glass: screen and backlight

===Antique TV=== Just a note that very old TVs are somewhat different to modern kit, especially in picture tube safety. Old picture tubes from the 1970s and earlier may lack a rimband, resulting in explosion if the tube's reinforced front glass is broken. Older pre-1960 picture tubes that use a separate plate glass shield to protect them are both fragile and explosive, and require treating with respect.

The EHT supplies in historic TVs can bite, but are normally safer than modern sets, and pose little risk to life. The exception is rare pre- war sets, which often contain a dangerous type of EHT supply.

==Washing machine==

• Mains filter sometimes stores mains voltage after the machine's unplugged.

• Power supply for controller stores mains voltage when unplugged

• Hand can get caught in mechanical parts

• Ill fitting spanners can slip, causing minor injury

==See Also==

• [[Special:Allpages|Wiki Contents]]

• [[Special:Categories|Wiki Subject Categories]]

[[Category:Appliances]] [[Category:Electrical]] [[Category:Fault Finding]] [[Category:Repair]] [[Category:Safety]] [[Category:TV]]

[The Medway Handyman]

I read 'filter' as being a water type filter on the incoming waters 'mains'. I guess you are using it to mean something electrical?

I've cut myself a couple of times on the sharp edges of the thin sheet metal used to make the backs.

[Rod]

Don't think that dropping 'EHT' in there is very helpful. I suspect many/most people reading would not know what that is - I am guessing extra high tension?

[Phil]

We used to dispose of lots of 'gassy' tubes by tapping the thin rear nipple on the tube with a long screwdriver. Never had one explode. The cautious among us used to cover it with bubble wrap - which used to get sucked inside!

[NoSpam]

... snipped

... snipped

All good but a couple of niggles:

- CRTs implode, not explode

- "Propped appliance can fall on child or pet" is not unique to a TV and is OTT. Better to say that they are surprisingly heavy and that the weight is concentrated towards the front (avoiding terms like CofG)

- May want to discuss degassing by crushing the nipple - but then you need to explain about covering the CRT in case of implosion

Dave

[mark]

I thought CRT TVs imploded.

mark

[Bob Mannix]

Back in the day, our local tip didn't sort anything - all was chucked from a height down onto concrete and every now and then a digger came and cleared the area. We used to take great delight in disposing of CRT tubes by hurling them onto the concrete so they did explode. Happy days....

[Andrew Gabriel]

Not to be confused with hyper tension, which results from the damn thing busting in the first place...

[Graham.]

The most serious omission IMHO is there is no reference to the hazards encountered when servicing, or connecting peripheral devices or test equipment to "live chassis" radio and televisions. These was mainly valve or hybrid (valve & semiconductor) sets, but some "all transistor" sets were also live chassis.

The chassis is supposed to be connected to the supply neutral, but even if you check the polarity it would be very unwise to work on it without an isolating transformer.

After about the mid 70's a different kind of live chassis is encountered. This time the chassis is connected to the -ve of the mains bridge rectifier. Consequently, the chassis is

*always* at half mains potential irrespective of plug polarity

With both these variants of live chassis, use an isolating transformer rated for the full continuous load (not an insubstantial piece of kit) whilst the back is off, and ideally, when in normal operation too.

Finally, the aerial socket will be isolated with built-in capacitors, ensure this hasn't been tampered with. Imagine getting a serious shock whilst in the roof!

[meow2222]

Thanks for the suggestions. version 2....

The main hazards in appliance repairs are summarized. There are always others that have the potential to occur too.

==Common risks== Some risks are common to all mains appliances:

• Shock if plugged in

• Basic wiring faults

==Microwave== These are one of the highest risk domestic appliances to repair. The EHT supply in particular tends to be fatal if touched, and has killed experienced technicians. (EHT = very high voltage.)

• EHT supply for magnetron

• Parts connected to EHT supply: LT magnetron supply wires, transformer, capacitor, rectifier

• Power supply capacitors sometimes store thousands of volts even when unplugged

• Misalignment or miswiring of interlock switches

• Failed shorting resistor when replacing [[fuse]]

• Microwave leakage after replacing a component

• Microwave leakage from rust holing

• Sharp edges

===Old microwaves=== Old machines from the beginning of the '80s and before can have:

• unsafe interlock system.

• carbon loaded rubber seals prone to coming adrift, permitting leakage

Some 1970s and earlier machines have power controls that switch the EHT directly, making all internal parts of the power control and associated wiring dangerous to touch until the EHT has been fully discharged.

Commercial machines from the 1960s and before are uncommon, but do exist, and in a minority of cases can continue cooking with the door part way open. These are hopelessly unsafe.

===Damaged microwaves=== The mains risks are:

• missing or damaged door grille cooks the user

• warped door

• missing seals on old machines

• damaged door hinges

• door misalignment

==TV== ===CRT TV=== TVs with bulky picture tubes are the other domestic appliance with assorted internal safety issues.

• Power supply capacitors store hundreds of volts even when unplugged. This voltage is distributed around some parts of the circuit board.

• EHT supply, stored by the picture tube when off

• Focus voltage, also stored

• HT supply, again often stored

• Propped appliance can fall on child or pet. CRT sets have their centre of gravity far forward, and often surprise casual repairers by toppling unexpectedly.

• Live chassis were common until the 1980s. Connecting external devices to these sets can shock, or in the case of headphones can kill. (Note that a minority of live chassis sets have headphone outputs driven by small isolating transformers, so existence of such a socket does not imply it has an isolated chassis.)

• Picture tube may explode if the unprotected thin rear of the tube is broken. (Breaking the nipple is the standard disposal method.) The large vacuum in the tube causes implosion, and the broken glass can bounce back out explosively.

===LCD TV=== Flat panel sets have much less internal safety issues.

• Power supply capacitors store hundreds of volts even when unplugged. This voltage is distributed around some parts of the circuit board.

• High voltage stored in lighting invertor

• Weak thin glass: screen and backlight

===Antique TV=== Just a note that very old TVs are somewhat different to modern kit, especially in picture tube safety. Old picture tubes from the 1970s and earlier may lack a rimband, resulting in a good chance of explosion if the tube's reinforced front glass is broken. Older pre-1960 picture tubes that use a separate plate glass shield to protect them are both fragile and explosive, and require treating with respect.

The EHT supplies in historic black & white TVs can bite, but are normally safer than modern sets, and pose little risk to life. The exception is rare pre-war sets, which usually contain a dangerous type of EHT supply.

==Washing machine==

• Mains filter sometimes stores mains voltage after the machine's unplugged.

• Power supply for controller stores mains voltage when unplugged

• Hand can get caught in mechanical parts

• Ill fitting spanners can slip, causing minor injury

• Sharp metal edges sometimes found on internal sheet metal

==Radio==

• Presence of mains in the cabinet ===Historic radio===

• Some pre-1970 valve radios known as universal sets used a live chassis arrangement. Connecting external devices to these is dangerous.

• Old valve radios may retain 100s of volts of charge in the reservoir capacitors when unplugged.

• Curtain burner sets are rare, the mains lead on these gets hot in use, with predictable results if not well ventilated along its full length.

==See Also==

• [[Special:Allpages|Wiki Contents]]

• [[Special:Categories|Wiki Subject Categories]]

[[Category:Appliances]] [[Category:Electrical]] [[Category:Fault Finding]] [[Category:Repair]] [[Category:Safety]] [[Category:TV]]

[Andrew Gabriel]

That just means the pieces shooting out in your direction came from the opposite side originally.

[PM]

Likewise "ill fitting spanners" is not the fault of the washing machine. (Unless they come with them?)

[Harry Bloomfield]

on 09/12/2008, [e-mail] supposed :

They are under vacuum and therefore implode though the result can be the same anyway.

[Graham.]

I'm not at all happy with the idea that breaking the area near the EHT connector is safe, in fact IMHO I can't think of a more dangerous way to destroy a tube.

The standard way of destroying the vacuum is to crack the *neck* with a small hammer ot the handle or a long screwdriver. It is easy to do because the glass is thin at this point, and the narrow neck ensures that the air rushes in comparatively slowly with a hiss rather than Earth-shattering KABOOM

[Graham.]

I strongly disagree with you, this is a wiki for the in-initiated, and both warnings should remain. Many modern CRT sets become very unstable if you attempt to remove the back cover with the set on its stand. The weight of the tube makes the whole escutcheon rake forward, along with the centre of mass.

As for the other point about ill-fitting tools in white goods, I would be in favour of removing the word "minor" in "minor injuries" as slashed wrists have been known to be fatal!

[Graham.]

Opps. Uninitiated, sorry

[Tim S]

Graham. coughed up some electrons that declared:

Agree, having dispatched several dozen at Imperial College this way.

Safety goggles, leather gloves, big long screwdriver, large hammer.

Bang screwdriver through the case centre rear of top, push in and feel for the neck of the tube, then a good tap with the hammer. Pop and a hiss later and job done. Screen turning light grey in patches is a secondary confirmation all went well.

Cheers

Tim

[NoSpam]

I don't wish to be confrontational, but following this line will lead to warnings that standing on one leg can lead to falling over and injury due to imbalance. A little bit of natural selection is a good thing.

[NoSpam]

When I were a lad the standard method was to crush the glass nipple in the centre of the connector. I don't think anybody is recommending breaking anything near the EHT connector.

[Mike]

What the hell is that? I've not had to do it for quite a few years but I've always used a reel to reel bulk tape eraser.