Electronics > Repair

"Double insulated, do NOT earth." Why?

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max.wwwang:
Earthing is a seemingly trivial but actually very complex topic. I used to think it was simple and I knew about this, but I knew virtually nothing, which was dangerous. Only recently, I got to know a little bit about this, including the topic of isolation transformer, through discussions in this forum and watching YouTube videos. But there are still some questions I don't have an answer to. Here is one.

On the casing of double-insulated appliances, often we see this on the label: "Double insulated, do NOT earth". I understand the reason why there is no NEED for earthing due to double-insulation, which means earthing does not provide more protection than without. But I don't understand why we should NOT (not need not) earth a double-insulated device. What's the RISK of doing this, beyond no more protection?

I have googled this topic but have not found a clear explanation.

brabus:
Earthing a double-isolated appliance is a no-go, simply because the producer has not defined a proper earthing contact for it. One could find a screw terminal, a hole in the metal case or even an actual earthing pole used in a different, earthed variant of the same appliance: please do NOT use them as earthing contacts. This would violate the manufacturer's specification regarding AC grid connection, potentially voiding the warranty and posing a risk for the end user.

max.wwwang:

--- Quote from: brabus on September 28, 2023, 08:41:05 am ---Earthing a double-isolated appliance is a no-go, simply because the producer has not defined a proper earthing contact for it. One could find a screw terminal, a hole in the metal case or even an actual earthing pole used in a different, earthed variant of the same appliance: please do NOT use them as earthing contacts. This would violate the manufacturer's specification regarding AC grid connection, potentially voiding the warranty and posing a risk for the end user.

--- End quote ---

Thanks. But I'm interested in what exactly the risk to the end user is.

brabus:
The risk is given by the fact that the manufacturer has not defined an earthing strategy for the product, so the consequences of forced earthing are unpredictable. In other words, the manufacturer deemed the risk of failure of the double insulation as far more unlikely than the risk of failure of the earthing system.

The most pesky circumstance is the failure of the earthing system on the AC side. If the main building earthing fails and another device is discharging current through the earth, the metal casing (that is supposed to be floating in a double-insulation device) would become energized, posing a risk to the end user.

A practical example: suppose you decide connect the secondary side ground of a phone charger to earth, although the charger is isolated. Everything goes well, you hold the cellphone in your hand and nothing happens. But if the building's main earth connection fails (it's not that unusual!), you WILL become the discharge path of any current flowing through the earth path. So, it is better to stay floating (i.e.: double-isolated) than becoming an active part of the earthing system.

SeanB:
The risk is failure of the internal parts, as often the internal insulation is not designed, or intended, to act as primary insulation, and can break down destructively with applied voltage, as the double insulation designation means the entire external insulation is designed to withstand mains, so internal parts do not need this.

For small power supplies however the output can be grounded by other methods, this just means your ground path may now have currents flowing, that were normally suppressed by the internal filtering of the power supply.

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