RS switch mode power supply

[hubsy]

Hi Guys,

Has anyone come across this SMPS from RS, 445-107.

Blurb says its class 1, because it has an earth pin.

I cannot get a circuit from the earth pin to the '-' of the output.

I have sent RS a question about this but have not had a reply.

Must fail it as a class 1 appliance.

 

[telectrix]

you should not get continuity from earth pin to -ve output. you should get it from earth pin to matal case (if it is a metal case). the output from the psu will be electrically separated from the input.

 

[hubsy]

Other smps that I have tested, that do not have the class 2 marks, do have continuity between the earth pin and the 'outer' of the DC plug.

Tested some 20+ over the past 2weeks and Class 1 do have continuity.

Cases are all plastic in class 1 and class 2.

Ref: COP 4thedition, page 92

 

[telectrix]

so that would indicate that some psu's are pelv and others are selv. i'm sure it's OK> RS don't sell rubbish.

 

[hubsy]

Yes RS equipment is good quality. Should be for the price.

But without somewhere to put the testers clamp, it fails as a class 1.

 

[telectrix]

it's a strange one for pat testing . the same with a lot of computer psu's 3 pin mains input but no metal to earth test. i have one on the desk now. not marked as classII. with IEC mains lead and says on it that it must be earthed. just got to accept it as it is.

 

[Grizzly01]

No, don't fail it as a Class I!
Just because it doesn't bring the earth connection all the way through to the output doesn't mean it's a fail.
Yes, some Class I power supplies do that, and others do not.
The earth connection will be there as a functional earth (it is a SMPS, after all!).

It is not marked as Class II, because it isn't Class II. It has an incoming earth. It cannot be Class II.
As it has an incoming earth, it is Class I.
However, this does not mean the earth has to reappear anywhere.
Do not fail it.
Make a note in the records.

The sooner many PAT testers get their heads round these simple concepts regarding PSUs, the better!

 

[Grizzly01]

But without somewhere to put the testers clamp, it fails as a class 1.

No, it does not!

Why are you suggesting this?

 

[telectrix]

visual inspection and common sense apply here , methinks.

 

[hubsy]

No, it does not!

Why are you suggesting this?

why the !!!!!!!!!

Class 1 is 0.1ohm plus the resistance of the cable to the appliance.

Tell me HOW to test the 0.1ohm measurement, without somewhere to test to?

I guess you have never had a faulty PSU kicking out 12volt tied to the 230v side instead of the neutral side. I have.

 

[Grizzly01]

Class 1 is 0.1ohm plus the resistance of the cable to the appliance.

Tell me HOW to test the 0.1ohm measurement, without somewhere to test to?

Simple: you don't.
But you also have to appreciate the difference between a earth provided for circuit protective purposes, and one provided for purely functional purposes.
Yes, in some PSUs, the earth will be doing both.
In others, only functional; the protection from electric shock being provided by Class II-like construction. Can't mark the appliance as Class II however, as there's an earth there!

Your assumption that all Class I PSUs will have direct connection between the earth pin and the -ve of the dc jack is erroneous. I can tell you that it is not always the case.

I guess you have never had a faulty PSU kicking out 12volt tied to the 230v side instead of the neutral side. I have.

And your point is...?

 

[hubsy]

voltage on 12 v circuit to earth was 218 - 230, instead of 0 - 12 volts.

mains shocks were being received from the 12v side

 

[Grizzly01]

Still don't see the relevance to your original point.

 

[hubsy]

if the negative of the output, on a class 1 psu, isn't tied to ground then its possible for the floating secondary to, under fault conditions, become connected to the high voltage on the primary side of the psu.

I am an Electronic Electrician, and have had this happen.

 

[Grizzly01]

I've no doubt you've seen it, but there's your assumption again.

Not all Class I power supplies have their -ve output tied to mains earth. This is a fact.
Their method of protection against electric shock will be provided by different methods.
And when you get one that is constructed in such a way, you have to face facts that you cannot perform an earth bond continuity test on it.
But that is no reason to fail it.
By failing such an appliance, the PAT testing technician is stepping beyond their remit, and by implication, beyond their own competence.

 

[hubsy]

update.
testing class 1 smps all from the same source.
All had earth to outer barrel connection, Except one tested yesterday.
failed it and replaced it.

took it apart and sure enough, one of the soldered lugs have parted company from the circuit board.
resoldered and its now working correctly. scrapped it because had to cut the case off.



btw I am a competent electronic electrician.