Author Topic: How safe is the 12V from a switched power supply?  (Read 1187 times)

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Offline Swake

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How safe is the 12V from a switched power supply?
« on: August 23, 2017, 03:54:21 pm »
How safe is the 12V from a switched power supply, this in particular related to lighting usage in a bathroom? (The question is not about the zone's one should respect or about a particular regulation in a specific country)

A power supply with a 12V potential difference is ok for a bathroom, but how to make sure that this 12V is effectively isolated from mains? My thoughts are that it will depend on the build-up of that power supply whether the output has a 'galvanic isolation' from mains or not. But then how can this be checked or tested? This is not as simple as doing a continuity test...

Sub question: How safe, again in regards to bathroom usage, are those small boxed power supplies that come with many LED lighting fixtures?


 

Offline ovnr

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Re: How safe is the 12V from a switched power supply?
« Reply #1 on: August 23, 2017, 04:01:58 pm »
Any normal SMPS plugpack is already isolated from mains, or you'd get a shock if you touched the output. Some grounded plugpacks connect the output ground to the mains ground, which is perfectly safe. Non-grounded plugpacks will generally contain a Y capacitor, which will present a small mains leakage current to the secondary side - but this applies regardless; if it's safe to touch in the living room, it'll be safe to touch in the bathroom.

So yes, a 12V PSU will be safe for a bathroom. If you try really hard, you can electrocute yourself with it, but it's no more dangerous than a couple 9V batteries in that regard.


LED lighting fixture PSUs may or may not be isolated. Do not count on them being isolated, or their isolation being sufficient for safety purposes.
 

Online Messtechniker

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Re: How safe is the 12V from a switched power supply?
« Reply #2 on: August 23, 2017, 04:14:33 pm »
My thoughts are that it will depend on the build-up of that power supply whether the output has a 'galvanic isolation' from mains or not. But then how can this be checked or tested? This is not as simple as doing a continuity test...
Get yourself an insulation tester (for example PEAKTECH 2695-2 or FLUKE 1503) and in the unpowered state check at its 500 V DC as well as its 1000 V DC setting insulation resistance between the mains side and the 12 V side. Insulation resistance should be well over 1 MegOhm.

Basically an insulation tester is a resistance meter but with a high test voltage  at a low current compared to common resistance meters where lower voltages are used.

In view of a lot of Chinese mains powered crap, investing in an insulation tester is generally a good idea.
Agilent 34465A, Hameg HMO1022, R&S HMC 8043, Voltcraft VC 940 and M-Audio Audiophile 192
 
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Offline tronde

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Re: How safe is the 12V from a switched power supply?
« Reply #3 on: August 23, 2017, 04:31:43 pm »

A power supply with a 12V potential difference is ok for a bathroom, but how to make sure that this 12V is effectively isolated from mains?

You should look for a power supply marked "SELV". It means Separated Extra Low Voltage or Safety Extra Low Voltage. These units have extra insulation. I guess the national regulations of your country specifies 12V SELV for use in bathrooms.
 

Offline fourtytwo42

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Re: How safe is the 12V from a switched power supply?
« Reply #4 on: August 23, 2017, 05:42:42 pm »
How safe is the 12V from a switched power supply, this in particular related to lighting usage in a bathroom?
Unfortunately CE labeling is no guarantee of the innards. Many power supplies with CE marks I have seen are positively dangerous, among other faults the transformer and pcb construction do not provide the required creepage and clearance dimensions.

Only buy from a reputable source MANUFACTURED in the EU/UK/USA then at least you have a chance, don't buy cheap rubbish from Ebay and the likes.
 

Online Messtechniker

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Re: How safe is the 12V from a switched power supply?
« Reply #5 on: August 23, 2017, 07:37:55 pm »
Unfortunately CE labeling is no guarantee of the innards. Many power supplies with CE marks I have seen are positively dangerous, among other faults the transformer and pcb construction do not provide the required creepage and clearance dimensions.

Only buy from a reputable source MANUFACTURED in the EU/UK/USA then at least you have a chance, don't buy cheap rubbish from Ebay and the likes.

Exactly my opinion.
Agilent 34465A, Hameg HMO1022, R&S HMC 8043, Voltcraft VC 940 and M-Audio Audiophile 192
 

Offline David Hess

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Re: How safe is the 12V from a switched power supply?
« Reply #6 on: August 23, 2017, 08:11:51 pm »
It should be very safe especially since you can attach ground to the negative output so a power supply isolation failure will trip any ground fault interrupter or blow the fuse or circuit beaker.  The negative output is likely already attached to ground but it could also be done externally just to be sure.

For low voltage lighting however, A switching power supply is *not* going to be more efficient than an AC step down transformer producing 12 volts AC.  And the switching power supply will produce more electromagnetic interference and be less reliable if that matters.
 

Offline Swake

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Re: How safe is the 12V from a switched power supply?
« Reply #7 on: August 23, 2017, 08:29:59 pm »
Sure a 'copper' transformer is nice, unfortunately most LEDs run of DC. The least it needs to make it work is a rectifier and some capacitor. That makes it much bigger in size and more difficult to hide. It would also require a not so common 9V transformer to end up at more or less 12VDC.

I've got a bunch of 100W 12V transformers laying around from the good old halogen times. Would consider putting a buck converter behind a rectifier/cap to throttle down to 12VDC   ;D
 

Offline xani

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Re: How safe is the 12V from a switched power supply?
« Reply #8 on: August 23, 2017, 09:00:46 pm »
Just please at least probe it to check for any funny business.

My boss few weeks ago asked why his LED power supply "kicks" him when he plugs it at home but not when he plugs it at work.

It had 3 wires for mains. It had ground wire. Just that, for some reason, case was connected to neutral instead of ground and apparently his circuit at home had swapped neutral and live. Couldn't see where ground wire actually went because it was potted
 


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