What constitutes double insulated?

I've bought a couple of pendant lights recently, and neither had the metal parts of the lamp earthed, and the cables were only two cores. Both lights have plastic bulb holders, but the wires are threaded through the metal parts to get to the lamp holders.

I wondered what physical features are needed to warrant the claim that something is double insulated. Is it simply an extra bit of heat-shrink?

Double insulated from what?

Lightium said:

Double insulated from what?

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"Double insulated" is a common term for a Class II electrical appliance. https://en.wikipedia.org/wiki/Appliance_classes#Class_II

Doesn't that answer your question?

Mike.

Pommie said:

Doesn't that answer your question?

Mike.

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I was looking for something about details of construction.

In a metal pendant light which has an earth tag on the metal ceiling part, I would have expected a 3-core cable earthing the metal of the hanging part. There was a 2-core cable, with joints inside a thin metal tubing, where the wires had been pulled through and round corners, with plenty of opportunity for the insulation to have been damaged. If there was any damage, it wouldn't be possible to see.

Diver300 said:

the wires are threaded through the metal parts to get to the lamp holders.

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I believe that would disqualify it from being classified as double-insulated.

For that, there would have to be another layer of separate insulation between the wire and the metal.

crutschow said:

I believe that would disqualify it from being classified as double-insulated.

For that, there would have to be another layer of separate insulation between the wire and the metal.

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The wires in the flex didn't go all the way to the lampholders. There was a soldered in-line connection, with tubing and heat shrink around it, inside the metal tubing, to some wires with stiffer insulation, which connected to the lampholders.

I don't know if that stiffer insulation would be counted as "reinforced" so that nothing else was required.

I've fitted a 3 core cable and earthed it all anyhow.

Pendant lights do not need an earth anyhow.. I also agree with Carl.. Heat shrink isn't thick enough to be classed as an insulator.. I would have used normal two core through the tube as "normal" two core is double insulated. Its only in case the tube rubs into the outer core..

Diver300 said:

The wires in the flex didn't go all the way to the lampholders. There was a soldered in-line connection, with tubing and heat shrink around it, inside the metal tubing, to some wires with stiffer insulation, which connected to the lampholders.

I don't know if that stiffer insulation would be counted as "reinforced" so that nothing else was required.

I've fitted a 3 core cable and earthed it all anyhow.

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You've now ensured that the unit no longer meets legal safety standards You have also assumed full responsibility for anything which might occur with the unit.

Double insulated refers mostly to the mains lead in this situation.

Ian Rogers said:

Heat shrink isn't thick enough to be classed as an insulator..

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I do not know if heat shrink counts as an insulator. I think it does.
I build transformers and use "triple insulated" wire for primary to secondary insulation. (very thin insulation. 0.4 mm x 3)
I use three layers of transformer tape for insulation (when I use normal wire). 1mm x 3

ronsimpson said:

I do not know if heat shrink counts as an insulator. I think it does.
I build transformers and use "triple insulated" wire for primary to secondary insulation. (very thin insulation. 0.4 mm x 3)
I use three layers of transformer tape for insulation (when I use normal wire). 1mm x 3

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Fair dues.. I have a megger sat here on my table, The only heat shrink I have here hasn't a very good insulation.. However! I'm just covering the soldered joint.. Double insulation would be heatshrink over the original pvc coating, so a bad test..

Well I just got a very mild shock from my garage door opener. It's a Horman Supramatic E3, and it is supplied with a two core cable that has a European 2 pin (non polarised) plug, plugged into one of these:-
https://uk.farnell.com/powerconnections/fcp-bk-3a/converter-plug-european-black/dp/3530940

None of the metal work of the garage door was earthed, and I measured around 90 V ac between any part of the garage door and the earth connection of the electrical system.

I realise that there was only about 0.1 mA of current available, probably from the Y-capacitors in the door opener, which is allowed for a double-insulated appliance. However it does seem a bit odd that an appliance likely to be connected to so much metal wouldn't be earthed.

Anyhow, I've now run an earth wire from the socket to garage door.

Does anyone else think that something like that should be earthed.

Diver300 said:

Well I just got a very mild shock from my garage door opener. It's a Horman Supramatic E3, and it is supplied with a two core cable that has a European 2 pin (non polarised) plug, plugged into one of these:-
https://uk.farnell.com/powerconnections/fcp-bk-3a/converter-plug-european-black/dp/3530940

None of the metal work of the garage door was earthed, and I measured around 90 V ac between any part of the garage door and the earth connection of the electrical system.

I realise that there was only about 0.1 mA of current available, probably from the Y-capacitors in the door opener, which is allowed for a double-insulated appliance. However it does seem a bit odd that an appliance likely to be connected to so much metal wouldn't be earthed.

Anyhow, I've now run an earth wire from the socket to garage door.

Does anyone else think that something like that should be earthed.

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No, the opener is double-insulated, so doesn't need to be earthed - that's the point.

The issue, if indeed there is one?, is that your large metal door wasn't earthed.

I have a similar one, Horman Supramatic HD. That also has only live and neutral terminals, no earth; its hard wired to its own MCB.

I've never noticed any leakage from it, but in my case there is no electrical connection between the door operator and the garage door metalwork - the door itself is a double fibreglass one, and the end of the operator track is attached to a wooden beam lintel over the door.

I do have the add-on UAP relay module, interlinked to my security/home automation gear; but again, I believe that to be totally isolated & using relay contacts.

I'll try to remember to check for any voltage on the operator metalwork sometime tomorrow - it's too cold and wet just now!


Earthing theoretically-floating metalwork is a tricky subject. In industry, pretty much any piece of metal anyone could come in contact with would normally be earth bonded - eg. metal control cabinets etc.

But, I've just searched to se if there are any regulations & found this, which seems counterintuitive to me.

Taken literally, such as machine tool beds or chassis should not be grounded as they are not "part of the electrical installation"??

I'd think anything that could become live due to an insulation failure should really be grounded / earth bonded, with an ELCB in the power feed to the equipment if people can come in contact with it.

It's taken me several years to notice the leakage.

The risk of the insulation failing on the door opener is slight. What is worrying is that quite a large bit of metal could be live and no-one would notice, so it could remain a danger for a long time. However the normal RCD protection would probably stop there being real danger.

There is the other risk that were someone to touch a live wire in the garage, it's better if the garage door (and any other stuff in the garage) were not earthed.

It was a bit worrying to get a tingle from the door when holding one of the other bits of metal in the garage. I'm happier knowing that if the insulation in the door opener brakes down, the RCD will trip immediately, rather than when someone touches the garage door.

I think most acdc SMPS suffer from common mode leakage capacitance from primary to secondary and the magnet wire and primary PVC counts as double insulation. But they must pass factory leakage tests.

Laptop metal cases have the same 100 uA or less leakage current and I easily detected this with my wrist on the laptop case with wet feet on grass while the charger was connected.

I suspect your examples use a flyback transformer to qualify for double isolated and it has leakage capacitance with a common mode voltage. It might only be a few pF but at 100kHz this 1 Mohm enough to cause this leakage. This will feel more like a burn on the wrist than a buzz, if my assumptions are correct..

Diver300 said:

It's taken me several years to notice the leakage.

The risk of the insulation failing on the door opener is slight. What is worrying is that quite a large bit of metal could be live and no-one would notice, so it could remain a danger for a long time. However the normal RCD protection would probably stop there being real danger.

There is the other risk that were someone to touch a live wire in the garage, it's better if the garage door (and any other stuff in the garage) were not earthed.

It was a bit worrying to get a tingle from the door when holding one of the other bits of metal in the garage. I'm happier knowing that if the insulation in the door opener brakes down, the RCD will trip immediately, rather than when someone touches the garage door.

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It's all a matter of degree - however, it's probably a requirement in the UK that large metal items like that are earth bonded anyway, in the way that all metal pipes need to be.

Safety wise?, it's all a matter of perception - under some circumstances earthing items increases safety, under other circumstances it makes it far more dangerous. However, for the general public, the decision has been taken for you, and legislation states what must be earthed.

The SMPS carrier unless active PF corrected with with CM chokes will have significant line modulated CM leakage.

A garage door acts as much larger capacitor than transformer C coupling to secondary.
For insulation to ESD we consider 1 Meg resistor as grounded surface. For a switched current carrier in a transformer we only care about the ac impedance to be >- 1 Meg and I suggest the garage door does not meet this requirement. It will be a far greater capacitance to PE
than the leakage capacitance to primary HV.

Adding a 1nF Cap to PE ground on V+ or V- which even at 50 kHz is 3 kohm will provide significant attentuation. I recommend you try that.

An appliance with double insulation, also known as a Class II electrical appliance is one which has been built in such a way that it does not require a connection to electrical earth (ground), and has 2 layers of insulation to protect the user.