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Topic: Is swtich mode power supply (SMPS) safely isolated?  (Read 764 times) previous topic - next topic

Watcher

Underwater pool lights traditionally use a mains/12v transformer which offers isolation from the earthing system. Older halogen lights were 12V AC powered which also made sense to install a transformer.

Newer LED lights however sometimes need 12V DC supply. Instead of just adding a rectifier (and hence the output would be unregulated 12V*sqr2) I am thinking of replacing the transformer with a regulated SMPS 12V DC like this one.
I am puzzled however regarding the safety of this power supply regarding mains isolation even though I know these also employ a transformer at higher frequencies.


Does anyone have any experience?

MarkT

Earth the output then you don't have to worry.

Typically they are isolated, but there is significant leakage current at the switching frequency (10's of kHz)
due to the much lower impedance of stray capacitances at frequencies in that range compared to mains
frequencies.

The leakage currents are not dangerous, but definitely give a tingle to wet hands, so you will want to earth
the output anyway.

If you require complete isolation including from mains earth, you need a mains frequency transformer with
physically separate bobbins, or use AC to low voltage DC converter with output earthed followed by
an isolated low voltage DC-DC converter to break the leakage current path (much less leakage at 12V
even at 10's of kHz)

Alternatively look for a SMPS designed for low leakage - using inter-winding screening and separate
bobbins on the high frequency transformer will have a big effect, but that costs more and is less
efficient.

Another thought: with a standard SMPS you could earth the output via a mains-rated capacitor to
siphon off the high frequency leakage current without a galvanic connection...
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Watcher

Thanks for the input MarkT.

The chassis of the SMPS is connected to system earth anyway. So any leakage current higher than 100mA will trigger and trip the local RCD protection. If that fails then the central house 300mA RCD will trip.

What I am worried about is the possibility of mains voltage somehow finding its way to the DC output of the power supply and eventually in the pool water giving people an electric shower!

Not too "confident" to also earth the DC output side (ie effectively connecting to the chassis) since I am not sure what effect will this have to the power supply itself.

Your suggestion for mains rated caps across the output and earth sounds interesting.
Wonder what would happen if the system earth is temporarily found at a raised potential ie. during a fault elsewhere...

MarkT

The chassis of the SMPS is connected to system earth anyway. So any leakage current higher than 100mA will trigger and trip the local RCD protection. If that fails then the central house 300mA RCD will trip.

What I am worried about is the possibility of mains voltage somehow finding its way to the DC output of the power supply and eventually in the pool water giving people an electric shower!
The SMPS leakage currents I talk about aren't dangerous, but you really don't want them in a wet
environment, its diconcerting at best.

Unless everything is isolated from the environment the earthing system is the key safety feature, it
should be regularly tested (I'd recommend a local RCD on the pool with a test button).  Then everything
can be bonded to this earth.  So long as the earthing system is intact any fault will trip the RCD or
if a hard short probably blow the fuse/trip the breaker too.

[ RCDs normally sense the difference in current through live and neutral, thus see any leakage from
live to earth as a discrepancy - choose the most sensitive RCD for the load if you can ]
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JohnRob

Hi,

I agree with MarkT's assessment up the point that the leakage in not dangerous.  While typically that probably true, when around water I believe things get more dangerous.

If I were putting such lights in my pool I would look for a medical grade power supply.  Maybe something like this:  https://www.jameco.com/Jameco/Products/ProdDS/2257651.pdf

They don't seem to be too much more expensive and personally the peace of mind is worth a lot more.  I would also purchase a supply from a more well known mfg.   Keep in mind the danger is really with the failure modes.  A medical rated supply is required to have more robust isolation so a failure is very unlikely to cause a safety concern.

You might also need to check your local building codes.

John
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Watcher


Quote
If I were putting such lights in my pool I would look for a medical grade power supply.  Maybe something like this:  https://www.jameco.com/Jameco/Products/ProdDS/2257651.pdf
Interesting approach but...
I havent seen anything in the datasheet about medical applications.
Does it have anything to do with one of the standards it complies with ?

Quote
Applications
Industrial automation machinery

Industrial control system

Mechanical and electrical equipment

Electronic instruments, equipments or
apparatus


Household appliance

Paul_KD7HB

I will bet your insurance company will require certified equipment installed by a licensed electrician, and that may require electrical permits and inspections.

Paul

srnet

I would assume that if anyone, using someone elses pool, got 'tingles' from the electrics, thay would get out of there pronto.


 
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JohnRob

I'm definitely not an expert on this subject but I have seen references to "medical grade" power supplies and with a little help from google I found this:


IEC spec


I believe the key points are the AC leakage and the physical layout of the board and parts, with an eye on failure modes.

Regarding local codes, I believe no two are the same.  Here in the US the NEC as a basis with local codes modified by some rules that I've never understood.

Also I've never heard of an insurance company denying a claim because the homeowner did their own work.   Now lawyers..... that is a different story.  Here in the US we are clearly litigation crazy :(

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Watcher

I am now inclined to raplace the transformer with a higher voltage one, ie 15V instead of 12V and then use a rectifier and a DC-DC back converter.

This way the required mains isolation is maintained by the transformer, but cant avoid transformer losses...

JohnRob

I am now inclined to raplace the transformer with a higher voltage one, ie 15V instead of 12V and then use a rectifier and a DC-DC back converter.
If it were only that easy.  You must choose a transformer with a split bobbin to insure the windings are separate.   A layered constructed transformer could short internally and connect the mains to the output.

Simply google "split bobbin transformer" and you will see numerous examples and offerings.  Again, for my pool I would choose a name brand transformer.

I realize this is probably frustrating for you, a seemingly simple task turns complex but I'm sure its worth the extra effort.


John
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Watcher

Quote
You must choose a transformer with a split bobbin to insure the windings are separate.   A layered constructed transformer could short internally and connect the mains to the output.
Yea..good point there.
Interestingly, the existing 12v transformer, installed by the original pool company,  is not a split bobbin  one!

hammy

You need to have approved equipment for this that meets appropriate standards .
Whatever was in first ...

Watcher

You need to have approved equipment for this that meets appropriate standards .
Whatever was in first ...
Anyone knows what exactly  these standards are?

JohnRob

It depends where you are.

In the US most local codes include the NEC (National Electrical Code) plus any local special requirements.

I looked at the 2012 NEC and could only find reference to UL 676.

It appeared to me the NEC was counting on a GFI for protection and I could find no mention of the transformer or power supply.



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