Electrical tingling question

[SimonH2]

Cheap chargers can have poor separation between mains and elv sides ...

Not just "cheap" stuff - and Apple's power supplies are not inexpensive
There tends to be some direct connection between primary and secondary - it may be "direct" by (eg) a resistor between one side of the secondary and primary to leak away any static, or it can be by way of AC filters which tend to be a pair of capacitors between the two input lines and "earth". Where there is no earth, then these capacitors form a voltage divider pulling that "earth" to around half the line voltage. So in the case of the Apple stuff, if you use an adapter/cable with an earth connection then the tingling goes away.

 

[Iggifer]

I think Apple has now rectified this issue somewhat - I used to get it all the time on my 2007 MBP and very occasionally on my 2010 model.

With my 2016 and now my 2017 (don't ask!) the issue is gone entirely - perhaps the move to USB-C PD has changed their charger design somewhat. I have noted that the two Magsafe chargers I have just looked at are not Class II, but the USB-C brick for my MBP is Class II. Whether the metal lug on the USB-C actually connects to anything, I'm not sure. Couldn't find a teardown and I'm not about to crack mine open!

 

[Astra99]

I would like to point out that the adapter shown in the third picture in EricMark's post is what is called a "charger adapter". This has a screw which locks the PSU in place (it will also accept a CEE 7/7 plug), and is covered when the adapter is inserted into a UK socket. It may, therefore, be used in the UK as it requires a tool to remove, unlike the first adapter (UK to Schuko) shown.

 

[JBR]

I get it with my iPad and iPhone on normal Apple chargers.
I can’t feel it if I gently run a finger over any other metal object whilst holding my phone (while charging), so if I am turning on a touch lamp for example.
Never worried about it.

I know the feeling! I've felt it on several electrical items, including this standard lamp next to me now. I am assured that it is only capacitive coupling.

 

[SimonH2]

Curious isn't it.
For as long as I can remember, standard advice has always been along the lines of "if you feel any sort of shock, there's something wrong/dangerous that needs to be investigated" .... but now it seems to be "quite OK" to get mild electric tingling from stuff. I think most of us here know the difference between a small leakage that's safe and something that's dangerous - but I strongly suspect that the average "man on the Clapham Omnibus" would be oblivious to the differences.
As an example ... Daughter No 1 called the other evening in a panic, "If I've spilled water in an extension lead, is it safe if I've switched it off at the wall ?" I told her to pull the plug out and she asked if it was safe to do so (the water was spilled on the sockets, not the plug). I don't think this level of knowledge is particularly out of the ordinary.

The other thing that comes to mind is ... what about the case when multiple items are connected together ? TV, DVD, VCR (yes we still use them for playing tapes !), STB, etc. If none of these have an earth then the leakage currents are going to add up - they'll probably all have filters with a capacitive voltage divider pulling the "chassis" up to half the line voltage. And I'm sure we've all heard of people seeing sparks when connecting/disconnecting leads between such bits of kit.
Looking across at our current setup, there's the TV, DVD, and VCR which are all 2-wire devices with no earth. Then there's a computer which is earthed - so the 3 other devices are earthed through the connecting leads.

 

[JBR]

The other thing that comes to mind is ... what about the case when multiple items are connected together ? TV, DVD, VCR (yes we still use them for playing tapes !), STB, etc. If none of these have an earth then the leakage currents are going to add up - they'll probably all have filters with a capacitive voltage divider pulling the "chassis" up to half the line voltage. And I'm sure we've all heard of people seeing sparks when connecting/disconnecting leads between such bits of kit.
Looking across at our current setup, there's the TV, DVD, and VCR which are all 2-wire devices with no earth. Then there's a computer which is earthed - so the 3 other devices are earthed through the connecting leads.

As it happens, only yesterday I was removing the plug of an aerial lead from one of my DVD recorders (under the telly) and thought I'd felt a sharp bit of wire protruding, sufficient to make me jump. There was no visible wire and so I concluded it was an electrical shock. If I recall correctly, my other hand was likely to have been in contact with the case of the unit. This was confirmed when I re-inserted the plug and found that the feeling had gone.

As you say, these things have only a two wire cable, no earth wire, and I can only assume they are designed to be safe. I hope!

 

[SimonH2]

Well yes, we hope they are designed to be safe
But in the case of input filters, if one of the caps goes short circuit ... there won't be enough current to "blow" it* but it'll pull the chassis to full mains voltage
* AIUI, components intended for this sort of application are designed to "fuse". So if a cap (between L&E or L&N) develops an internal short, it will "fuse" without drawing enough current to heat up and explode. In addition, the encapsulation is designed to rupture gracefully rather than allow pressure to build up first.

 

[bernardgreen]

When a device is supplied by mains without an Earth then the mean ( average ) potential of all the circuitry will be somewhere between Live and Neutral.

Am isolated 12 volt DC output from the device will ( should ) have 12 volts between the two output terminals but they will have AC potentials different from Ground ( Earth ) potential. The AC potential is induced by capacitive coupling and thus is a high impedence and unable to supply a significant current. The AC potential will depend on how the devce is built and which way a reversible mains plug is inserted.

This applies to a magnetic wound transformer and shows how the induced voltage relative to ground is affected by the arrangment of the windings on the transforner and the stray capacitive coupling between them.

In the case of an SMPS ( electronic transformer ) there is a high frequency transformer the secondary of which will be close to one side of the incoming mains supply.

If B is Live then the induced potential of the ELV output will be close to Neutral. If B is Live and A is Neutral then the the stray capacitive coupling between the windings will induce a mean potential on the ELV output that is close to the Live potential above ground.

 

[aptsys]

Of course, neither of those illustrations are representative. You would not find a standard transformer with a linear wound 230V primary - the windings are always layered.

On the SMPSU, you would not get away with a completely floating ELV output - there are always some components between the two sides of the transformer to allow the common mode currents to be coupled back to the primary side.

 

[bernardgreen]

the windings are always layered.

If the bottom layer ( closest to the core ) of the primary is the Live end then the top layer of the primary is at Neutral potential. If the secondaries are wound on top of the primary then the secondaries are closer to Neutral than to Live.

there are always some components between the two sides of the transformer to allow the common mode currents to be coupled back to the primary side.

Can you expand on that ?, it implies that an SMPS can never provide a fully isolated ELV supply, something that a magnetic transformer can do.

 

[SimonH2]

They can, SMPSUs normally use an opto-coupler (LED and optical sensor) in the feedback loop for total isolation. But the issues of coupling apply, and it's common for some point of the output to be earthed to some degree or other.

 

[bernardgreen]

and it's common for some point of the output to be earthed to some degree or other.

but without an Earth on the incoming mains the ELV output cannot be "Earthed" at source. Only if it is connected to Earth via other equipment or a person's fingers and body capacitive coupling to Ground / Earthed items can the output be earthed to some degree.

 

[JBR]

I wonder whether in my experience (post #21) the metal ferrule on the aerial connector (obviously connected to the screening) could have provided an earth.

Another question: would it be sensible to connect the casing of our DVD recorders to earth? There are screws on the back that can provide a connection to the metal casing and an earth conductor could extend to the earth pin on the plugs.

 

[JohnW2]

Another question: would it be sensible to connect the casing of our DVD recorders to earth? There are screws on the back that can provide a connection to the metal casing and an earth conductor could extend to the earth pin on the plugs.

You are presumably talking about Class II equipment since, if it were Class I, the metal casing would be required to be earthed.

Assuming it is Class II, then does it not really come down to the standard argument - that creating any extra bits of touchable earthed metal increases hazards? In other words, whilst it may well get rid of tingles, it would slightly increase the chances of 'real' (potentially injuring or lethal) electric shocks.

Kind Regards, John

 

[bernardgreen]

there won't be enough current to "blow" it* but it'll pull the chassis to full mains voltage

If the chassis is not Earthed then the filter capacitors should not be connected to it. A floating chassis would not provide a parth for currents from the filter so there be little if anything to be gianed by connecting filter components to the chassis.

no earth wire, and I can only assume they are designed to be safe. I hope!

They are ( or should be ) designed to be as safe as is economically viable.

Ensuring isolation by separating the mains winding from the secondary winding(s) by using split or twin bobbins is safer than winding them both in the same bobbin but more expensive.

If a transformer built with a split bobbin overheats and the bobbin melts then a short circuit between mains and secondaries is possible. A mains to secondary short circuit is even less likely to happen if a dual bobbin transformer overheats and bobbins melt.