Class 2 product?

[Shpark]

If you have say a metal based lamp and it’s still a class 2 product. What would happen if the exterior somehow became live. There’s obviously no path to earth?

 

[pc1966]

In that case it is a hazard as anyone touching it and another earthed object will get a potentially fatal shock (depending on any RCD protection in place). With no earth to divert the current and blow the fuse, etc, it will stay live.

Generally most electrical systems are designed to be safe against a single point of failure. So with "double insulated" equipment there should be precautions in the design stage that no practical single failure could render external metalwork live.

Thus it should be virtually impossible for this to happen. But it can, if someone has modified it badly or it is a rubbish fake/knock-off product that was never done properly. This last aspect has been seen with fake Apple chargers, etc, that are actually dangerous due to bad design/manufacture to sell below Apple's usurious price.

 

[Shpark]

So a poorly modified, class 2 lamp becomes live in an old house, with BS 3871 mcb’s and no RCD protection...you’re very much at risk?

 

[snowhead]

Assuming the lamp was manufactured with the metal base then the risk is no greater than the flex to an electric kettle failing and the live touching a metal saucepan.

 

[pc1966]

So a poorly modified, class 2 lamp becomes live in an old house, with BS 3871 mcb’s and no RCD protection...you’re very much at risk?

Yes.

But that is also true of faulty/badly modified Class 1 products as well.

 

[radiohead]

There are a lot of metal class 2 lights now with a silly little enclosed connector and a rubber tube to protect the cable entering the connector. Fine if you have one bell wire to terminate, impossible with 3x 1.5mm T&E's. Unless I can pull the cables into the ceiling void and box them with a single cable down, I invariably ignore the class 2 bit and earth them.

 

[Shpark]

Yes.

But that is also true of faulty/badly modified Class 1 products as well.

But if this class one product fails and makes the metal exterior live, surely with it having an Earth point, this will draw current closing the EFLP and blow either the plug top, mcb/rcd?

 

[DPG]

But if this class one product fails and makes the metal exterior live, surely with it having an Earth point, this will draw current closing the EFLP and blow either the plug top, mcb/rcd?

Yes, but remember the class II appliance has to effectively have 2 failures before a shock happens. Theoretically anyway.

 

[pc1966]

But if this class one product fails and makes the metal exterior live, surely with it having an Earth point, this will draw current closing the EFLP and blow either the plug top, mcb/rcd?

Depends on how it fails.

A lot of PA equipment for bands was class 1 with earths, and yet you would still see musicians being killed in the pre-RCD days due to damaged cables causing the earth to fail and then another fault happened possibly months later.
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Trying to prevent double-faults is kind of the idea behind PAT testing. Not that you catch deadly equipment before an accident (nice if you do) but you find things with partial damage or a single-fault before it can progress on to a fatality or fire.

 

[Shpark]

Yes, but remember the class II appliance has to effectively have 2 failures before a shock happens. Theoretically anyway.

What would be the 2 failures on a class 2?

 

[pc1966]

What would be the 2 failures on a class 2?

You need 2 failures in either class:
Class 2 - double insulation fault
Class 1 - earth disconnect and an live to chassis insulation fault

 

[Shpark]

Depends on how it fails.

A lot of PA equipment for bands was class 1 with earths, and yet you would still see musicians being killed in the pre-RCD days due to damaged cables causing the earth to fail and then another fault happened possibly months later.
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Trying to prevent double-faults is kind of the idea behind PAT testing. Not that you catch deadly equipment before an accident (nice if you do) but you find things with partial damage or a single-fault before it can progress on to a fatality or fire.

Yeah I get you. I think I’d feel a lot more safe earthing a product though, than solely relying on someone’s home ability to double insulate.
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You need 2 failures in either class:
Class 2 - double insulation fault
Class 1 - earth disconnect and an live to chassis insulation fault

Hmm. From what I understand, where the flex is terminated is double insulated. In that if the live comes lose somehow, it is contained in a insulating area. However when then flex eaters the light, they tend to use a rubber grommet to stop the flex fraying on the chassis. Say if the lamp cable was somehow tripped on and the grommet loses it position, which I’ve seen, after a while of shaking on a metal entry, it could potentially wear through the cable exposing the live.
I know this is far fetched, but it will happen eventually to someone. Then you face an exposed live conductor without an earth

 

[DPG]

To be honest there are good and bad examples of both class I and class II devices. Both can be perfectly safe though if made correctly.

 

[Julie.]

Yeah I get you. I think I’d feel a lot more safe earthing a product though, than solely relying on someone’s home ability to double insulate.
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Hmm. From what I understand, where the flex is terminated is double insulated. In that if the live comes lose somehow, it is contained in a insulating area. However when then flex eaters the light, they tend to use a rubber grommet to stop the flex fraying on the chassis. Say if the lamp cable was somehow tripped on and the grommet loses it position, which I’ve seen, after a while of shaking on a metal entry, it could potentially wear through the cable exposing the live.
I know this is far fetched, but it will happen eventually to someone. Then you face an exposed live conductor without an earth

Take a step back and look at it a bit wider!

In essence a double insulated thing has two independent layers of insulation, each capable of providing sufficient insulation on its own; imagine this is like taking the roasting tin out of the oven with the potatoes and beef sizzling away at 250 degrees+, one layer of the tea towel would be sufficient.

Would you fold the tea towel over "just in case"?

thats the concept behind double insulation.

the alternative is just one layer and rely on the earth/cpc connection always being there, yet how many times have we come across dodgy cables or extension leads - wires hanging out of the plug - with the earth/cpc actually not connected?

in this case you would be relying on the earth when it isn't there at all!

For many years equipment was supplied with bare ends - stripped down for 30-40mm and you fitted your own plug, unfortunately this meant people would just connect it as-is, so when pulled the earth being the furthest away would be the first to come out - yet class one absolutely needs the earth/cpc!

in the same vain a lot of people would make extension leads for table lamps etc with only line & neutral - if these ended up being used for class one stuff then again no earth/cpc connection at all.

so this is why double insulated became the standard for portable tools and other devices wherever possible - overall it has far fewer failure points than the equivalent class 1, not in terms of the tool/device itself but in terms of the wider picture.

I would agree, if you can absolutely absolutely absolutely guarantee that the earth connection is in place all the time under all conditions, then it is probably the best, but anything other than such absolute guarantees then class 2 tends to be preferable.

note, class 2 includes both double insulated and reinforced insulation, where a single layer of insulation is so much more than needed, it is the equivalent of double.

 

[Shpark]

Take a step back and look at it a bit wider!

In essence a double insulated thing has two independent layers of insulation, each capable of providing sufficient insulation on its own; imagine this is like taking the roasting tin out of the oven with the potatoes and beef sizzling away at 250 degrees+, one layer of the tea towel would be sufficient.

Would you fold the tea towel over "just in case"?

thats the concept behind double insulation.

the alternative is just one layer and rely on the earth/cpc connection always being there, yet how many times have we come across dodgy cables or extension leads - wires hanging out of the plug - with the earth/cpc actually not connected?

in this case you would be relying on the earth when it isn't there at all!

For many years equipment was supplied with bare ends - stripped down for 30-40mm and you fitted your own plug, unfortunately this meant people would just connect it as-is, so when pulled the earth being the furthest away would be the first to come out - yet class one absolutely needs the earth/cpc!

in the same vain a lot of people would make extension leads for table lamps etc with only line & neutral - if these ended up being used for class one stuff then again no earth/cpc connection at all.

so this is why double insulated became the standard for portable tools and other devices wherever possible - overall it has far fewer failure points than the equivalent class 1, not in terms of the tool/device itself but in terms of the wider picture.

I would agree, if you can absolutely absolutely absolutely guarantee that the earth connection is in place all the time under all conditions, then it is probably the best, but anything other than such absolute guarantees then class 2 tends to be preferable.

note, class 2 includes both double insulated and reinforced insulation, where a single layer of insulation is so much more than needed, it is the equivalent of double.

I don’t think that’s how double insulation works. It’s not literally a cable with 2 layers of insulation. It’s what surrounds the terminations inside the appliance.

 

[David Prosser]

I don’t think that’s how double insulation works. It’s not literally a cable with 2 layers of insulation. It’s what surrounds the terminations inside the appliance.

That is exactly how it works. You have a active/live part surrounded by the basic insulation and then by supplementary insulation.
Look at the cable supplied with class 2 equipment it usually simple flex, inner and outer insulation.

 

[Shpark]

That is exactly how it works. You have a active/live part surrounded by the basic insulation and then by supplementary insulation.
Look at the cable supplied with class 2 equipment it usually simple flex, inner and outer insulation.

Yeah so a class 2 appliance has a 2 core cable which has an outer sheath and then the insulation for both live and neutral, separate ofc! So 1 layer of insulation. It is then terminated inside an insulate box/container/whatever. So one insulation is the cable and 2 is the termination box

 

[David Prosser]

No the cable has two layers of insulation.

 

[Shpark]

No the cable has two layers of insulation.

The sheath is not insulation. Insulation is the brown and blue layers over the copper. GCSE PHYSICS - Mains Electricity - What is Double Insulation? - What is the Symbol for Double Insulation? - Which Appliances are Double Insulated? - GCSE SCIENCE. - https://www.gcsescience.com/pe38.htm

 

[Darkwood]

Double insulated cable is slightly misleading term although not exactly incorrect. The insulation surrounds the conductor directly, the sheath is the environmental protection and yes it has insulating qualities that may match the actual insulation itself but is also will be design to cope with the expected environmental conditions to which it is normally installed.

I use specialised cables a lot in machinery and the sheath of which there may be several layers are all designed to ensure the insulation is protected and maintained and is not subject to anything that may break it down.

You are not technically incorrect to use the term double insulated as that is one of its attributes but if you were to do my job then that term would not suffice and they would be clueless to which cable I wanted.