Does this power supply have an isolated output?

[kubeek]

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.

No one says that a device that has no earth connection cannot pass EMC. Only that once it was designed in such way that it needs ground connection, you should not cut that ground wire.

Please look at the post scriptum simulation I added in the previous post.

 

[kubeek]

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

Almost universally all old analog scopes have metal cases. All new ones even though the case is plastic have at least the probe ground and usually a spare ground post. Telling someone not to touch what should be at ground potential so they don´t get shocked is madness.
Also if you plug an extension cord into an outlet that has no ground, then this possibly lethal voltage will be on grounded cases of everything else connected to it, like the shield on a USB connector of a PC etc.

 

[Flyback]

So you are saying that if i snip the earth wire out of the plug of a modern scope, then plug it in, then touch the scope probe's ground tip, then i will definetely be electrocuted?
........i actually don't think so, i think that scope that zapped me (which i previously described) had been dropped hundreds of times, and i was just seeing if someone had a similar story, and could proove that it was earth snipping that caused it.......

Also, imagine that you are testing a mains operated SMPS that does have a connection for earth ground.
.........if you are going to scope the waveforms at the primary side of this SMPS, how do you do it?...what equippment do you use?

Thanks for your waveform, i am still analysing it, and thinking about this.

 

[KeepItSimpleStupid]

You just use the scope in a-b mode or use the a+b mode with b inverted or we go back to an isolation transformer on what your servicing or use a battery powered scope.

For a 1000 W 13.56 MHz tube transmitter, I won't and can't snip the third prong because the load is firmly grounded unless it's a dummy load. I also cant do it if the device (another one) requires 70 Amps, 3 phase to operate.

 

[kubeek]

So you are saying that if i snip the earth wire out of the plug of a modern scope, then plug it in, then touch the scope probe's ground tip, then i will definetely be electrocuted?
........i actually don't think so, i think that scope that zapped me (which i previously described) had been dropped hundreds of times, and i was just seeing if someone had a similar story, and could proove that it was earth snipping that caused it.......

I didn´t say that. Sniping out ground right at the scope gives you only one fifth of the current I posted before, which will give you a mild shock. I still don´t see any reason apart from being cheap why would one want to get 60V shock each time he touches a scope ground.
However when you take this one step further and snip the ground right at the wall socket or inside the extension cord, then plug in a few devices with Y caps to ground, the current through you will rise with each device connected.

For measurement at mains I would either use A-B mode like KISS pointed out. Or you use isolation transformer (prefarably to lift the device under test, not the scope) and measure with appropriate precautions, like power-off, change connection, power-on plus a GFCI breaker on the isolated line. Or you simply bite the bullet and buy the differential probes, if the company makes power supplies or other mains equipment and needs to measure it accruately, it should simply be considered a must-have.
I have no idea what country are you from, but where I live the revision technician who checked the power cables with lifted ground every year and let them pass the inspection would spend a few years in jail.

 

[Flyback]

For a 1000 W 13.56 MHz tube transmitter, I won't and can't snip the third prong because the load is firmly grounded unless it's a dummy load. I also cant do it if the device (another one) requires 70 Amps, 3 phase to operate.

.......agreed....considering for the discussion though, 230VAC single phase mains operated consumer domestic equipment.

I'd confer in that i'd feed the smps through an isolation transformer....................you tend to find that isolation transformers have the earth taken through to the output socket though.........and therefore, you have no choice other than to snip the earth wire out of the mains plug that you use to plug in the isolation transformer.

I wouldnt use a battery scope............they keep discharging too often.

 

[KeepItSimpleStupid]

kubeek:

I think your the first person to understand the ~60 VAC problem. Nearly everybody else is really baffled. The problem cost me a CPU back in the early 80's when computers cost a lot of money. It also cost 16 man hours for a PHD and a graduate student's time and they could not find the problem.

 

[Flyback]

I didn´t say that

....i just read what you wrote just after you wrote the above....and it is in fact what you said...as follows

I didn´t say that. Sniping out ground right at the scope gives you only one fifth of the current I posted before, which will give you a mild shock. I still don´t see any reason apart from being cheap why would one want to get 60V shock each time he touches a scope ground.

....so, please may i repeat the question............

So you are saying that if i snip the earth wire out of the plug of a modern scope, then plug it in, then touch the scope probe's ground tip, then i will definetely be electrocuted?

if the company makes power supplies or other mains equipment and needs to measure it accruately, it should simply be considered a must-have.

You asked what country.....it is England...........now known as the laughing stock of the industrial world for its inability to manage manufacturing industry due to strikes which crippled itself..........and hence the reason why companies wont buy pricey diff probes.........English consumer electronics companies cannot compete with those from the Far East, and so managers here want to cut costs any way that they can, because they know we are never going to make much money, as that is made by the Far Eastern electronics companies.

 

[kubeek]

you tend to find that isolation transformers have the earth taken through to the output socket though.........and therefore, you have no choice other than to snip the earth wire out of the mains plug that you use to plug in the isolation transformer.

And what is wrong with that? You want the metal cases of anything to be tied to ground at all times. In any circuit, the only connection between a neutral and ground should be at the breaker panel. See this **broken link removed** The AC power is isolated, but the case still remains grounded. This way you can safely put the ground lead of your scope anywhere in the circuit and measure what you want. If the device you measure has a connection between neutral and ground it´s design is wrong and it should not pass any regulation.

 

[kubeek]

Electrocution means death (execution) by electricity. Unless you have a heart condition or pierce the skin on your hands and let the current bypass the skin resistance, with one hand at the scope and the other at a grounded object, it should not kill you, that is why regulations set maximum ground current in a working device. This doesn´t mean it will be a pleasant experience.

 

[schmitt trigger]

.

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.


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.

s

low power SMPS can gset away without a ground because the common mode current is below the thresholds mandated by regulatory agencies.

Diff probes are not expensive. Google Probemaster model 42332. Only US$332. Those greatest companies should be able to afford them easily, a piece of cake.
What sort of slave-drive companies you are talking about, which would fire an employee for requesting safe equipment? In all of the Western countries, you could sue the company for wrongful termination and win.

 

[Flyback]

In all of the Western countries, you could sue the company for wrongful termination and win.

..............i very much doubt this............in fact, i am certain i could not (i wouldnt want to anyway as i know what i am doing with mains, and i wouldnt be unsafe) . In any case, even if i did "win", it wouldnt be a big pay out, and i'd never be able to find work again in the industry. Also, "the boys" would likely come and visit me...and not for tea and scones.

Anyway, here's one reason why everybody used to float the scope (cut out the earth of the plug) in a huge lighting company and a huge TV company..............

.....................when you are testing a mains SMPS, you feed the SMPS like this..................

Mains wall socket > Autotransformer > Power meter > SMPS.

....you cannot have an isolation transformer in that path as you would affect the reading of the power meter (due to parasitic properties of the isolation transformer)............so therefore, you must have your isolation transformer in the scopes supply, and then have no choice but to cut out the earth connection, otherwise you will get a "bang".

.........................
Thanks for the probemaster diff probe link

**broken link removed**

.......trouble is, its only having bandwidth of 25MHz...........not so great for seeing drain voltage spikes on flyback smps..................also, just look at the dreadful 4mm connectors, we should be using BNC to connect up to smps circuit, not those horrible noisy separate wires that exist on the probemaster 4232.

 

[kubeek]

It think you could have the isolation transformer in front of the autotransformer. What are the parasitic properties of an isolation transformer?

 

[Flyback]

What are the parasitic properties of an isolation transformer?

.............just plug one in on no-load and feel how hot it gets.....thats what i mean........as well as the stray inductances which will affect the power factor measurement of the power meter.

Anyway, this business of the metal cases of mains equipment that have had the earth snipped out of the plug rising up to dangerous potentials....................and even more so when a multi outlet has many grounded devices plugged in together and the multi outlet has no earth ground connection because its been snipped...................

..................this sounds hard to understand.....and to be honest , hard to believe.

I am going to try and demonstrate this, as i think it can't happen.

You are speaking of the input Y caps, and their connection to earth ground, and you are saying that because of this, if the earth ground is snipped then the centre point of those y caps is going to assume a dangerous potential.................

............i dont see how, i mean it should be the centre point of hot and neutral, and should be zero volts, as its AC at the inlet, before the mains rectifier in the product.

I am going to buy two Y caps, and connect them in series, and connect mains hot to one end , and mains neutral to the other end, and then i'll scope probe the centre point that you speak of.....................i bet it is just at zero volts.

........do you agree.?

.........it wont have floated up to a dangerous potential....( i will probe the centre point with respect to earth ground)

Then this will proove that snipping out the earth doesnt matter.

 

[kubeek]

Think about it as a resistor voltage divider. One end on neutral = 0V, the other on phase - 120Vac. What will be the voltage on the mid point? 60Vac.
The same thing happens when you replace the reisitors with capacitors, you again get a voltage divider. End of one cap will be firmly at 0V (with respect to ground), the other will be swinging to +/- 120V. The midpoint will be right in the middle, so it will be swinging to +/-60V.

 

[Diver300]

There are two things that seem to have been missed in this discussion.

1) While Schuko plugs, such as:- only have two pins, but they still have an earth connection. The earth connection is either the recess in the plug, or the two metal strips on the edge, depending on what type of socket it is plugged into.

2) Having no earth connection won't make the case of any equipment have a lethal voltage on it, unless there is some other fault. So equipment without the earth connection that it was designed for could work for years with no problems. However, an internal fault could make the equipment dangerous. The capacitors to earth that help reduce noise could easily fail to short circuit. If that happens on a power supply where there is no earth, the output would then be at mains voltage and there would be a big risk of an electrical shock.

Earthing correctly is a bit like fusing things correctly. It is often only there to protect against something else going wrong.

 

[JimB]

Flyback

This may sound rather harsh but I feel that I must say it.

At numerous points during this thread you have demonstrated a distinct lack of basic knowledge and understanding of some very basic electrical principles.
And now it appears that not only DON'T you understand that an unearthed chassis will float to half supply voltage due to stray capacitance and/or EMC supression capacitors, you seem to resulutely REFUSE to understand, and are determined to try and prove that the rest of the world is wrong.

Please let us know how you get on with your test, one way or the other.

For someone who claims to have worked for a major TV and lighting manufacturer (you dont appear to state what it was that you did there), this simple stuff seems to be somekind of a Thorn in your side.

JimB

 

[kubeek]

The capacitors to earth that help reduce noise could easily fail to short circuit.

Y rated caps are used where there is a possibility of electrical shock when, and they are self-healing - will not fail shorted, typically from phase and neutral to ground. https://www.electro-tech-online.com/custompdfs/2013/01/EvoxRifaRFIandSMD.pdf

 

[Flyback]

OK granted about the mid point of the caps..............i now see why i missed this, its because recently i have worked on smps's without the earth ground coming to them.....so there were no y caps like that...the y caps used just went from primary ground to secondary ground.

So, coming to the original question, would there be any likelihood of the KERT KAT5VD power supply having its output common connected to earth ground?......i mean, why would anyone want to connect the secondary of a smps (such as in the kert kat5vd ) to earth ground?.........is it likely?

.....i am wondering if it could get done so that radiated emissions would be less likely to emit out of the product?

Is it a stupid question to ask if the output common of the kert kat5vd power supply is connected to earth ground?

(I cant get into work to get the ohmmeter on it to find out, as we are snowed out till thursday)

................................
Here is an axample of a mains smps of 120W with no earth connection.......so it is strange that some need earth and some dont

..................there is a chassis ground, which is not earthed, which is very unusual, page 2.............

https://www.electro-tech-online.com/custompdfs/2013/01/AND8207-DPDF.pdf

 

[schmitt trigger]

Most, if not all, lab grade supplies that I have seen have a shorting bar between earth (ground) and the common output. In other words, it allows you the option to choose whether to ground it or not.

However, this is true for North American market supplies. It appears that you are not located in NA, and the safety and/or emissions requirements will be different, and it may be that it is mandatory for it to be always grounded.

Perhaps another poster that lives in the EU could shed some light.