Does this power supply have an isolated output?

[Flyback]

Does this power supply have an isolated output?


KERT KAT5VD:
**broken link removed**

 

[ronsimpson]

It is not clear to me.
When a supply has "red", "black" and "green" connections it is clear that BLACK is not connected to GREEN.
When a supply only had RED and BLACK it is not clear abut assumed BLACK is not connected to ground.

 

[Flyback]

sometimes one presumes the best way to find out is to plug the blessed thing in, then probe its output positive with reference to earth ground ...access to earth ground being gotten by bringing a wire out of a mains plug?

if the output reads the same as the meter on the PSU, then it is not isolated from earth ground?

 

[ronsimpson]

Use a ohm meter. Measure from Green wire on the plug to the black post.

 

[Flyback]

Thanks, but i am not sure if that is a safe way to do it...because obviously the PSU has to be plugged in and ON......and that means that if it is an isolated PSU, then it may have floated up to a very very high voltage, and i thus may blow up the Ohm Meter?

.having said that, if the ohm meter DOES blow up.....then we definetely know that it must have been an isolated power supply?

 

[JimB]

Thanks, but i am not sure if that is a safe way to do it...

No, it is a perfectly safe way to do it.

because obviously the PSU has to be plugged in and ON......

No, you can do this test with the PSU disconnected from the mains.

and that means that if it is an isolated PSU, then it may have floated up to a very very high voltage, and i thus may blow up the Ohm Meter?

No, whatever gives you that idea?

.having said that, if the ohm meter DOES blow up.....then we definetely know that it must have been an isolated power supply?

No, totally erroneous assumptions.
Nothing is going to blow up.

JimB

 

[Flyback]

OK thanks, .by the way, the KERT KAT5VD has a 2 pin mains plug............so that means that it can't possibly have an output connected to earth ground....or does it?.........isnt mains neutral connected to earth ground in the vicinity of the installation/building.................so wouldnt the output common be connected to input neutral?

This is quite something, i am always being told not to cut off the earth connection from a three pin plug...........well hold on.........all European mains products are 2 pin, so they have exactly that, a plug with no earth connection.....so why am i always being told (eg told by eng-tips forum members) that cutting the earth wire of a plug is a bad thing?

 

[JimB]

OK thanks, .by the way, the KERT KAT5VD has a 2 pin mains plug. ...........so that means that it can't possibly have an output connected to earth ground....or does it?.........

No it does not.
It has a Shuko plug which has two pins for the power and a side contact for the earth connection.

isnt mains neutral connected to earth ground in the vicinity of the installation/building.

Yes, usually.

................so wouldnt the output common be connected to input neutral?

No, I hope not!
There is a transformer which provides isolation between the incoming mains and the DC output.

This is quite something, i am always being told not to cut off the earth connection from a three pin plug

Quite right!

...........well hold on.........all European mains products are 2 pin, so they have exactly that, a plug with no earth connection

Nonsense!
Some European plugs are two pin, but there are lots of three pin ones, like the Shuko on that Italian PSU.

.....so why am i always being told (eg told by eng-tips forum members) that cutting the earth wire of a plug is a bad thing?

I guess you have no idea why there is an earth connection there.
The earth connection is to connect all exposed conductive parts to earth, so that in the event of a failure which results in a connection between the exposed conductive bits and the live mains, then there is a path for the fault current which results in a blown fuse or a tripped RCD, so isolating the mains supply and making the equipment safe.

JimB

 

[Flyback]

Some European plugs are two pin, but there are lots of three pin ones, like the Shuko on that Italian PSU.

.that surprises me, maybe i am just getting old, but i thought all French German, Dutch, Spanish and Italian mains plugs are two pin?

The schuko plug surely cannot be classed as a three pin plug...............there is no earth pin...............the bit of earth on the schuko plug is nowhere near the metal casing of the product.....................i cant see how fault current would flow from the hot and somehow jump through the air and get to the bit of metal on the schuko plug thats in the wall.?

 

[ChrisP58]

OK thanks, .by the way, the KERT KAT5VD has a 2 pin mains plug............so that means that it can't possibly have an output connected to earth ground....or does it?.........isnt mains neutral connected to earth ground in the vicinity of the installation/building.................so wouldnt the output common be connected to input neutral?

This is quite something, i am always being told not to cut off the earth connection from a three pin plug...........well hold on.........all European mains products are 2 pin, so they have exactly that, a plug with no earth connection.....so why am i always being told (eg told by eng-tips forum members) that cutting the earth wire of a plug is a bad thing?

If something is designed such that it depends on the ground pin to be safe, then cutting the pin off makes it unsafe. But many things are designed using double insulation, so that they are safe without a ground pin.

If there is a ground pin, you need to assume that it is there for a reason.

That being said, there are many products where the ground pin is there not so much for safety, but is needed to meet EMI requirements. Many desktop switching power supplies are like that.

As for European power connectors, there are both grounded and ungrounded plugs. I expect that all modern sockets have a ground contact, but the plugs may, or may not, have a third contact.
**broken link removed**

disclaimer: I have never been to Europe so I have no personal knowledge of the plugs used there

 

[Flyback]

Thanks, i know what you mean by EMC....but i cant understand how an Earth ground connection has any help for it................lets face it, most European SMPS's feed off two pin plugs, and they dont have earth connection and they dont have EMC problems.

I have been to France several times, and to Germany , and to Croatia, and they all use two pin plugs everywhere.

In England , it is 3 pin plugs, but all of the supermarkets in England are full of three pin to 2 pin connectors so people can go to europe and use the sockets there.

I gaurantee you that Non English European countries only use 2 pin plugs, and an earth connection is never taken to the product.

Just go to H Gee's shop in Cambridge UK, and watch a torrent of European students/ people coming in to by 2 pin to three pin adapters because all there European products have two pin plugs only.

 

[KeepItSimpleStupid]

Yep, earth is there for a reason. RFI/EMI is one of them. Protective ground is another.

Isolating the device under test, isolate the instruments or do differential measurements.

here is a really BAD thing to do. Cut off the ground on an outlet strip, then plug in grounded devices. If there is an RFI filter in one of the connected devices from the outlet strip, it could raise the case of the devices to about 1/2 way of the line voltage. Not good. Not too many people understand how this happens.

Take a microwave and a sink. You would like the sink faucet to have the same reference potential as the oven. That what ground does.

The latter trend is the use of GFCI's and double insulation to deal with the "people factor". i.e. cutting off the ground plugs.

 

[Flyback]

Thanks......."outlet strip" means plug?

it could raise the case of the devices to about 1/2 way of the line voltage

...i wonder if this was why i kept getting shocks off a scope with the earth chopped out of its plug?.........whenever i touched the probe ground, i got a shock.

Also, when i worked on flat screen tv power supplies, it was standard for old timer engineers to snip off the 1 meg resistors that connected between primary and secondary on the tv smps, on the TV's that were being worked on by juniors................the juniors were using isolation transformers, and were surprised when they kept getting shocked by touching the power supply in one place.......but it was becasue the old timers had snipped off the 1 meg resistors.......the 1 meg resistors discharge the charge that gets induced into the secondary common via the isolation transformer interwinding capacitance.............................the shock that the juniors got was a capacitive shock going right through the isolation transformer insulation and back to the mains.

Anyway, Kert rang me up to acknowledge my question but would not tell me the answer.they said theyd ring again.

anyway

Isolating the device under test, isolate the instruments

......as can be seen from my TV days, isolating the instruments etc with an isolation transformer is not always much help....those shocks certainly used to wake me up when i kopped a few of them...before i noticed the snipped resistors.

it could raise the case of the devices to about 1/2 way of the line voltage

..but if its got a plastic case , then we're OK?....or if we make sure not to touch the case, then we're ok?


......................
sorry to persist, i've already been kicked off engtips forum for saying this, but i worked at one of the worlds biggest lighting companies, and one of the worlds biggest electric drive companies, and one of the worlds biggest TV companies, and we always snipped the ground out of plugs..everybody did it.

for some reason, the mains isolation transformers often used to have earth ground connected to the secondary....so we'd just snip out the earth from the plug that plugged the isolation transformer into the mains............

.......we had this contractor come in and tell us it was bad................but he couldnt proove it......he just seemed to be quoting what others had told him....................................he couldnt actually draw a schematic going back into the power system to show us what was wrong..................the lighting company had been earth lead snipping for 25 years, so they were hardly going to change..........especially since there is not one single coherently written article on the worlds surface that tells succintly why snipping earth out of plugs is a bad idea.

 

[KeepItSimpleStupid]

You want to work on a 100 kV 0.1 A or a 12 kV 1.5 Amp power supply with the third prong snipped? I don't and won't. An electron gun that has 30 kW of power isn't something to sneeze at.

Take a look here: http://www.cor.com/Series/PowerLine/AQ/ Note the two caps that are grounded in the center. Suppose that ground isn't connected to earth, but to another instrument?

That cap sort of acts like a voltage divider using it's internal resistance (which is high), but a voltmeter will detect this relative to a real ground.

If something like RS232 is communicating to something else with a "REAL GROUND", the devices sees 60 VAC instead of zero. People feel it too. Two researchers had this issue and spent 8 hours together trying to find it. I found the problem in about 5 minutes.

 

[Flyback]

Suppose that ground isn't connected to earth, but to another instrument?

I dont see how that could get connected to another instrument?...its inside the product case..how can it get connected to something else?

You want to work on a 100 kV 0.1 A or a 12 kV 1.5 Amp power supply with the third prong snipped? I don't and won't

.....Agreed, but i should have said that i'm talking about 230VAC work.

I know what you mean about high kv...................i once worked at a TWTA place, and an exposed 13KV terminal was making a loose screw in an earthed enclosure, 1 metre away, flash over to the earthed enclosure once per second......we coul dhear this tick tick tick.......then we saw the spark........

.....also, i approached a 13kv live terminal once with my arms folded so i wouldnt touch the 13kv............my forearm came within 40cm of the 13kv terminal and i could feel the muscle in my arm vibrating about.

But for me, cutting the earth out of 230VAC plugs is no problem, and the only thing to do if you dont have a mains isolation transformer................................you just have to be careful not to touch any metal casing etc.......

 

[KeepItSimpleStupid]

I dont see how that could get connected to another instrument?...its inside the product case..how can it get connected to something else?

If it's a TV try the coax antenna terminal. If it's an amplifier, try the signal generator used to fix it.

If the outside case is metal, then it may have 60 VAC (assuming 120VAC mains) on it. So, you touch your scope that's mining it's business and not even on, but it's a solid ground and you get shocked.

 

[Flyback]

OK , but as far as snipping the earth off of a scope's plug..............i think thats ok, because most scopes dont have a metal case.......................and, as i mentioned earlier, if you get shocked by touching the scopes probe ground, then to be honest, just dont touch the probe ground or anything connected to it..........................This induced voltage, i dont think it forms a very stiff ground, its just a kind of induced voltage, and as i often experienced, its more of a light snap than a shock when you touch it...............i take your point about examples, but concerning oscilloscopes and test equipment, that doesnt apply so much because the scope wont be connected to anything else.

I have still yet to hear of a good reason for not doing plug earth snipping on scopes.

As i say, i used to work at probably the biggest western electric drives company, and we all had our own mains plug leads with the earth snipped out..........one time, this guy was on holiday, and another guy came in borrowed his "earth snipped mains lead".......................when the guy came back off holiday he accidentally used a non earth snipped lead, and his circuit exploded...............................................................he told the boss, and the guy who took the lead got given punishment of tidying up the engineering office.

Is anyone going to come in and say that this huge enormous electric drives company, with huge sales, dont know what theyre doing?

 

[fernando_g]

The reason for an earthing (grounding) plug is both safety AND electromagnetic compliance.
Most, if not all modern electronic equipment produce EMI, both radiated and conducted. If said equipment is going to be used in North America, the European Community or other similar countries, those countries have very strict regulations with respect to EMI compliance.
To meet those regulations, the equipment uses, among others things, common-mode filters, which include a capacitor from each AC line to earth. Removing the earth disables in large part the filter's performance.
There is more. The capacitive reactance allows 50/60 hz current to flow, which is safely shunted to earth. Removing the earth in the plug has this current nowhere to go, except into the body of an unsuspecting operator or to another equipment which is connected to the unearthed equipment. This means a shock or erroneous readings. Removing the earth is dangerous and probably illegal.

If you require to measure a voltage whose common is elevated in voltage above earth, then use a differential scope probe. Google the term.

To your last sentence...... Even in the biggest, largest, most prestigious corporations, universities and organizations, there will be arrogant individuals which make stupid decisions.

 

[kubeek]

I have still yet to hear of a good reason for not doing plug earth snipping on scopes.

I think all those reasons were already mentioned: 60 or 120VAC on case(s), EMC compatibility.. For example, if you have a 5-socket cord with cut-off ground and have in there 5 devices with EMI filters, 2x4.7nF Y-caps each, you can get about 1mA fault current through your body. That doesn´t look very healthy, especially with nice path to ground through other hand.
Can you tell a good reason to do it?

EDIT: fault RMS current through resistor on 230Vac line, 2k to 200k logarithmic sweep with x1.2 difference, mouse pointing at line for 53.2kohm.

 

[Flyback]

Even in the biggest, largest, most prestigious corporations, universities and organizations, there will be arrogant individuals which make stupid decisions.

..your absolutely right....but regarding snipping off the earth out of plugs, it was not individuals, it was all engineers.....everybody.

......Plenty of mains powered SMPS's dont have an earth wire brought to them, and yet they still manage to pass EMC, and common mode filters are still possible in mains powered SMPS's, even if there is no earth connection.

Differential probes are ridiculously expensive, and need expensive calibration each year..................most companies have very few of them or none at all........engineers are simply told to "get on with it".

So we seem to have debunked that earth connection is necessary to pass EMC, becasue 2 wire smps's with no earth connection still pass EMC.

.....And as far as safety goes, if a product has a plastic casing, and the mains live comes into contact with the inside of that case, then you wont be electrocuted if you touch that plastic case at the outside of it.

I can tell you that i worked for some of the world's biggest companies in the fields of lighting electric drives and TV sets...........and earth snipping is standard practice amongst all engineers................if an engineer went to the boss and refused to do a job because he had no isolation transformer or no differential probe, he would not be working as an engineer in these companies for long..........he would be transferred to work in "administrative duties" or basic work, etc etc.

Have you seen the cost of differential probes?....its horrendous.

I remember a contractor coming in and telling our boss off for having us all do earth snipping.............they sacked him at the end of the week.

So , i would ask again, are these huge organisations (not individuals, because its not just induviduals) that i worked in doing it wrong?

60 or 120VAC on case(s),

..................i've never seen a scope with a metal case........they all have plastic cases