Is swtich mode power supply (SMPS) safely isolated?

Using Arduino General Electronics
#9

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 :(

#10

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...

#11

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.

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

#12

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!

#13

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

#14

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

Anyone knows what exactly these standards are?

#15

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.

#16

Regulations are Country specific - I’m in the UK

#17

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.

#18

jackrae: 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.

#19

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

#20

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.

#21

jackrae: 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.

#22

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.

#23

srnet: 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.

#24

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)

#25

jackrae: 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.

#26

polymorph: 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?

#27

polymorph: 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.

#28

Paul__B: No,it wasn't "parasitic" capacitance it was the "Y" capacitors on the interference suppression module.

Good point, likely correct.

Watcher: 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.