Connecting Power Brick 0V to metal enclosure.

[ecat]

Just a quick question on safety.

As I understand it, the output from most switch mode power bricks is floating. Are there any safety concerns connecting the brick's 0V, or any other output line, to the metal case of a hand-held device?

[Richard Crowley]

ASSUMING you are talking about a proper power brick (or wall wart) that has inspection certification (CE, UL, et.al.), I would have confidence that the output (either positive or negative) is sufficiently isolated from the power mains that it is inherently safe.

[ecat]

Thank you for the quick reply.

My concern is two fold, the inherent isolation but also what happens if the user has the metal case in one hand and touches some other metal cased and earthed mains appliance?

[Richard Crowley]

That is exactly what I mean by "inherently safe".  That is one of the things that all those inspection certifications test for.

[ecat]

And that's all there is too it? Excellent, thanks again.

I was remembering when a colleague had his battery powered laptop connected to some industrial cabinet by a rs232 cable, battery was running low, he connected the power brick and things went pop. Didn't  want to risk popping people.

[lewis]

You have to be a bit careful, on some power supplies you can measure ~150V between output and mains earth with a good digital multimeter, assuming the power supply is a class II type (not earthed - double insulated). The current is very low, but might be felt by some. The standards bodies set limits on 'Touch current'; BSEN 60990 shows some ways to measure it.

[Zero999]

And that's all there is too it? Excellent, thanks again.

I was remembering when a colleague had his battery powered laptop connected to some industrial cabinet by a rs232 cable, battery was running low, he connected the power brick and things went pop. Didn't  want to risk popping people.

It's possible the RS232 output isn't isolated and connecting it to the earthed industrial equipment caused a short circuit.

You have to be a bit careful, on some power supplies you can measure ~150V between output and mains earth with a good digital multimeter, assuming the power supply is a class II type (not earthed - double insulated). The current is very low, but might be felt by some. The standards bodies set limits on 'Touch current'; BSEN 60990 shows some ways to measure it.

Yes, under certain conditions, it's possible to feel a tingle, especially if the current passes through a small area of the body with a high density of nerve endings, such as the fingers. This is only a problem if the appliance is used in a hospital, where there's a risk the current could pass straight through the heart, in which case this is easily remedied by connecting it to an isolation transformer.

The main hazard to electrical equipment is ESD, that 150V is enough to destroy the gate of a MOSFET, although proper design can protect against this.