Arduino Forum

Using Arduino => General Electronics => Topic started by: Watcher on Oct 20, 2018, 11:28 am

Title: Is swtich mode power supply (SMPS) safely isolated?
Post by: Watcher on Oct 20, 2018, 11:28 am
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 (https://www.ebay.co.uk/itm/DC12V-Top-Quality-LED-Driver-Power-Supply-Transformer-LED-Strip-Light-MR16-CCTV-/162283920873?ssPageName=STRK:MESE:IT).
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?
Title: Re: Is swtich mode power supply (SMPS) safely isolated?
Post by: MarkT on Oct 20, 2018, 11:44 am
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...
Title: Re: Is swtich mode power supply (SMPS) safely isolated?
Post by: Watcher on Oct 20, 2018, 12:03 pm
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  (https://en.wikipedia.org/wiki/Residual-current_device)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...
Title: Re: Is swtich mode power supply (SMPS) safely isolated?
Post by: MarkT on Oct 20, 2018, 08:59 pm
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  (https://en.wikipedia.org/wiki/Residual-current_device)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 ]
Title: Re: Is swtich mode power supply (SMPS) safely isolated?
Post by: JohnRob on Oct 20, 2018, 10:32 pm
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
Title: Re: Is swtich mode power supply (SMPS) safely isolated?
Post by: Watcher on Oct 21, 2018, 09:58 am

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
Title: Re: Is swtich mode power supply (SMPS) safely isolated?
Post by: Paul_KD7HB on Oct 21, 2018, 06:14 pm
I will bet your insurance company will require certified equipment installed by a licensed electrician, and that may require electrical permits and inspections.

Paul
Title: Re: Is swtich mode power supply (SMPS) safely isolated?
Post by: srnet on Oct 21, 2018, 06:37 pm
I would assume that if anyone, using someone elses pool, got 'tingles' from the electrics, thay would get out of there pronto.


 
Title: Re: Is swtich mode power supply (SMPS) safely isolated?
Post by: JohnRob on Oct 21, 2018, 11:27 pm
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 (https://global.ihs.com/doc_detail.cfm?document_name=IEC%2061558-2-16&item_s_key=00524617&csf=TIA)


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 :(

Title: Re: Is swtich mode power supply (SMPS) safely isolated?
Post by: Watcher on Oct 22, 2018, 05:16 pm
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...
Title: Re: Is swtich mode power supply (SMPS) safely isolated?
Post by: JohnRob on Oct 22, 2018, 07:50 pm
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
Title: Re: Is swtich mode power supply (SMPS) safely isolated?
Post by: Watcher on Oct 22, 2018, 08:45 pm
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!
Title: Re: Is swtich mode power supply (SMPS) safely isolated?
Post by: hammy on Oct 22, 2018, 09:24 pm
You need to have approved equipment for this that meets appropriate standards .
Whatever was in first ...
Title: Re: Is swtich mode power supply (SMPS) safely isolated?
Post by: Watcher on Oct 22, 2018, 10:05 pm
You need to have approved equipment for this that meets appropriate standards .
Whatever was in first ...
Anyone knows what exactly  these standards are?
Title: Re: Is swtich mode power supply (SMPS) safely isolated?
Post by: JohnRob on Oct 23, 2018, 12:37 am
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.



Title: Re: Is swtich mode power supply (SMPS) safely isolated?
Post by: hammy on Oct 23, 2018, 08:39 am
Regulations are Country specific - I'm in the UK
Title: Re: Is swtich mode power supply (SMPS) safely isolated?
Post by: jackrae on Oct 23, 2018, 09:28 am
Slightly off-topic but if your house has either 100mA or 300mA RCD protection then it effectively offers no personal protection against electrocution.  These currents are far more than required to cause fatality.
Title: Re: Is swtich mode power supply (SMPS) safely isolated?
Post by: Watcher on Oct 23, 2018, 12:43 pm
Slightly off-topic but if your house has either 100mA or 300mA RCD protection then it effectively offers no personal protection against electrocution.  These currents are far more than required to cause fatality.
True if these currents were to pass from the body. The 300mA RCD  value is calculated  based on the touch voltage which corresponds to around 50v if i am not mistaken.
Title: Re: Is swtich mode power supply (SMPS) safely isolated?
Post by: hammy on Oct 23, 2018, 04:30 pm
Any use ?  The problem will be that any third party inspection is going to look for approved equipment not home made , so he doesn't have assess it and create personal liability for himself.  ( inCourt -"why did you approve this " A " it's marked as suitable and meeting approved standards"
Verses.. A" I considered it was safe "
Which is followed by " I see , what is your experience with x,y,z..are you qualified in designing ABC .")

Same would apply to yourself , if you kill anyone ... you are responsible for the design and test of whatever you put in, and need to have the correct ( provable ) expertise . There may also be an issue giving advice on forums on such matters , unsure.


https://static1.squarespace.com/static/5544eff8e4b067ba8b4230e6/t/579a17823e00bedf2acb83b2/1469716355553/Electrical-WIRING-SWIMMING-POOLS.pdf
Title: Re: Is swtich mode power supply (SMPS) safely isolated?
Post by: JohnRob on Oct 23, 2018, 06:17 pm
Quote
Any use ?  The problem will be that any third party inspection is going to look for approved equipment not home made
The curious thing about this topic is I could not find any information that would suggest what that approval might consist of.   Suppose the installation used a Medical Grade power supply (or any power supply for that matter) which was UL, CE, etc approved;  would that be considered "approved equipment".

The little research I did on this topic (NEC 2012 and UL 676) concentrated on bonding of metal parts and use of GFI / RCD devices.

Also the OP mentioned the original lighting (hopefully "approved") did not even have a split bobbin transformer.


Title: Re: Is swtich mode power supply (SMPS) safely isolated?
Post by: srnet on Oct 23, 2018, 08:51 pm
Slightly off-topic but if your house has either 100mA or 300mA RCD protection then it effectively offers no personal protection against electrocution.  These currents are far more than required to cause fatality.
Why would anyone install a 300mA RCD ?

My shed, built by me, was wired to the then current UK 'regulations' (not actually legal requirements) which specified 30mA RCDs. That was maybe 20 years ago.

Title: Re: Is swtich mode power supply (SMPS) safely isolated?
Post by: polymorph on Oct 23, 2018, 08:56 pm
Many years ago, I was manager of a video arcade. When the power supplies failed, we replaced them with generic switch mode power supplies.

However, I found out that without grounding, just the leakage from parasitic capacitance in the power supply was enough so that the chassis would float at 1/2 of the line voltage, 60V here, and with enough current to give you a palpable shock. I discovered this by touching the chassis of two games, one with a broken ground lug on the cord, the other properly grounded.
Title: Re: Is swtich mode power supply (SMPS) safely isolated?
Post by: jackrae on Oct 23, 2018, 10:06 pm
Why would anyone install a 300mA RCD ?

My shed, built by me, was wired to the then current UK 'regulations' (not actually legal requirements) which specified 30mA RCDs. That was maybe 20 years ago.


Maybe a "repossessed" one from an industrial site/employer in the mistaken belief that they were enhancing their home safety.  That said, the original poster did state the RCDs in question were 100mA and 300mA.
Title: Re: Is swtich mode power supply (SMPS) safely isolated?
Post by: JohnRob on Oct 24, 2018, 01:25 am
In the US the GFI (RCD) trips at a difference of 5 ma.


The one thing all the specifications I found essentially say bond everything to ground, with heavy wire (i.e. #12 AWG)

Title: Re: Is swtich mode power supply (SMPS) safely isolated?
Post by: Watcher on Oct 24, 2018, 07:08 am
Maybe a "repossessed" one from an industrial site/employer in the mistaken belief that they were enhancing their home safety.  That said, the original poster did state the RCDs in question were 100mA and 300mA.
Central house RCD protection installed just after the electricity meter is rated at 300mA.
Individual mains outlet circuits are protected by 30mA RCDs.


The pool supply distribution board, which then supplies pool pumps, pool light, etc, is protected by 100mA RCD.
Title: Re: Is swtich mode power supply (SMPS) safely isolated?
Post by: Watcher on Oct 24, 2018, 07:11 am
Many years ago, I was manager of a video arcade. When the power supplies failed, we replaced them with generic switch mode power supplies.

However, I found out that without grounding, just the leakage from parasitic capacitance in the power supply was enough so that the chassis would float at 1/2 of the line voltage, 60V here, and with enough current to give you a palpable shock. I discovered this by touching the chassis of two games, one with a broken ground lug on the cord, the other properly grounded.
Did you ground the metal enclosure of the arcade game and power supply box or the low voltage side of the power supply?

All metal enclosures shouldn't have been grounded according to local regulations anyway?
Title: Re: Is swtich mode power supply (SMPS) safely isolated?
Post by: Paul__B on Oct 24, 2018, 12:35 pm
However, I found out that without grounding, just the leakage from parasitic capacitance in the power supply was enough so that the chassis would float at 1/2 of the line voltage, 60V here, and with enough current to give you a palpable shock.
No,it wasn't "parasitic" capacitance it was the "Y" capacitors on the interference suppression module.

The 300 mA RCD is there to trip on earth faults in equipment, such as burn-out of a metal-jacketed element where the element shorts to the jacket.

The 30 mA rating is specified to protect against people actually connecting themselves from line to ground.  This is a historically later consideration in the regulations.
Title: Re: Is swtich mode power supply (SMPS) safely isolated?
Post by: polymorph on Oct 24, 2018, 06:07 pm
No,it wasn't "parasitic" capacitance it was the "Y" capacitors on the interference suppression module.
Good point, likely correct.

Did you ground the metal enclosure of the arcade game and power supply box or the low voltage side of the power supply?

All metal enclosures shouldn't have been grounded according to local regulations anyway?
The metal chassis inside the game is connected to the ground on the AC cord. This includes the metal doors for the coin acceptors and coin box. And probably the metal shafts on the joysticks.