Output voltage of shaver socket, seems high

[Chris Green]

My B-I-L (who is an electrician) has come across an oddity. A shaver
socket that he has installed produces an output voltage that measures
as 280 volts. He has asked about it on another forum and there they
are speculating that the isolating transformer has a 1.1:1 step up to
compensate for voltage drop when a shaver is plugged in.

It all sounds a bit odd to me, I think it's more likely that the meter
that says the voltage is 280 volts is mis-reading for some reason.
Does anyone here have any knowledge of such things?

(We don't currently know the manufacturer, nor has the '110 volt'
output been measured)

--
Chris Green
·

[Scott]

That was my understanding from a thread some time ago. Something to
do with the output floating and nominal voltage being produced only
when the correct load is applied. Someone also suggested that a
multi-voltage shaver should always be plugged into the 115V socket to
remove the risk of over-voltage.

[NY]

"Chris Green" <c...@isbd.net> wrote in message
news:lg1cli-6...@esprimo.zbmc.eu...

It would be interesting to know

a) the voltage of the mains at a 3-pin socket nearby

b) the voltage at the shaver socket when a typical shaver is plugged in


It is quite possible that the meter is mis-reading. I've got a Kasa
mains-monitoring plug in a socket by my desk and I glance at the voltage
from time to time. I've seen readings of around 230-250 V, varying over the
course of the day. But never anything as high as 280V.

I've just plugged our washing machine into another of these, to record
cumulative energy over the course of the day to see how much energy it uses
typically. Various other similar devices are recording voltages of around
230-240V, but this one is reporting 180 V (!) and the power, voltage and
current readings do not agree according to P = IV. When the washing machine
has finished and I've got the washing on the line, I'm going to investigate.
It's far more likely to be a dodgy metering unit than a socket that has a
50V drop compared with everywhere else: 50V with a current of 9 A is 450 W
being dissipated in a resistive loss somewhere it it's true ;-)

[David Wade]

Real RMS AC voltages depend on the shape of the wave. If something is
distorting the wave form it could well affect the reading

Dave

[John J]

If BIL really IS an electrician and is satisfied his meter isn't telling lies he should have telephoned his supplier emergency number and reported the situation. 280volts will be regarded as a dangerous situation and an urgent response team visit. Is there a preponderance of solar panels on roofs around the site? Of course solar inverters SHOULD shut down under over voltage conditions.

[alan_m]

On 17/05/2022 09:50, NY wrote:

> It is quite possible that the meter is mis-reading. I've got a Kasa
> mains-monitoring plug in a socket by my desk and I glance at the voltage
> from time to time. I've seen readings of around 230-250 V, varying over
> the course of the day. But never anything as high as 280V.

The meter may be assuming a sine wave output.


--
mailto : news {at} admac {dot} myzen {dot} co {dot} uk

[Clive Arthur]

On 17/05/2022 09:04, Chris Green wrote:

My /guess/ is that at very low secondary currents - almost none really -
the transformer core hysteresis is significant. As the very small
primary current changes polarity, there's a period when it's not
magnetising the core, then the hysteresis is overcome, so the magnetic
field 'jumps' as it changes direction.

This will affect the wave shape which may affect the measured secondary
voltage and will affect the actual voltage howsoever defined in some way.

Basically, no load isn't a fair test.

--
Cheers
Clive

[John Rumm]

The shaver socket is likely the type with its own isolation transformer.
I get the impression from reading the OP that thus voltage is present on
its secondary, not on its input. i.e. this is not the mains supply
voltage that is high.


--
Cheers,

John.

/=================================================================\
| Internode Ltd - http://www.internode.co.uk |
|-----------------------------------------------------------------|
| John Rumm - john(at)internode(dot)co(dot)uk |
\=================================================================/

[Mark Carver]

Yes, I'd like to see what it is under load (even though of course the
load is tiny). If it had been me, I'd have done that, and measured the
voltage across the primary/input.

What provoked the Sparkie to measure the voltage in the first place ?

[Chris Green]

John Rumm <see.my.s...@nowhere.null> wrote:
> >
> > If BIL really IS an electrician and is satisfied his meter isn't
> > telling lies he should have telephoned his supplier emergency number
> > and reported the situation. 280volts will be regarded as a dangerous
> > situation and an urgent response team visit. Is there a preponderance
> > of solar panels on roofs around the site? Of course solar inverters
> > SHOULD shut down under over voltage conditions.
>
> The shaver socket is likely the type with its own isolation transformer.
> I get the impression from reading the OP that thus voltage is present on
> its secondary, not on its input. i.e. this is not the mains supply
> voltage that is high.
>

Exactly correct! :-)

--
Chris Green
·

[NY]

"alan_m" <ju...@admac.myzen.co.uk> wrote in message
news:jehbhe...@mid.individual.net...

> On 17/05/2022 09:50, NY wrote:
>
>> It is quite possible that the meter is mis-reading. I've got a Kasa
>> mains-monitoring plug in a socket by my desk and I glance at the voltage
>> from time to time. I've seen readings of around 230-250 V, varying over
>> the course of the day. But never anything as high as 280V.
>
> The meter may be assuming a sine wave output.

On further testing, this particular Kasa is under-reading the supply
voltage, even under no-load conditions, in a variety of sockets. Other Kasas
and a digital multimeter all give similar voltage readings, so the one I
happened to use for the washing machine is dodgy. The question is: is it
under-reading by a constant factor? Time to test it with known loads (eg 1x
and 2x 60 W tungsten bulb, compared with another Kasa that is giving
sensible voltage readings) and see if the power readings are linear. If so,
I can at least work out a conversion factor by which I multiply any
power/energy readings.





I'm surprised that a shaver outlet has an output with such high impedance
that it only reads true when it has a shaver as a load, and has to be
designed to over-read on no-load, so that it reads true with the load it is
designed for.

[Fredxx]

On 17/05/2022 12:07, Clive Arthur wrote:

> On 17/05/2022 09:04, Chris Green wrote:
>> My B-I-L (who is an electrician) has come across an oddity.  A shaver
>> socket that he has installed produces an output voltage that measures
>> as 280 volts.  He has asked about it on another forum and there they
>> are speculating that the isolating transformer has a 1.1:1 step up to
>> compensate for voltage drop when a shaver is plugged in.
>>
>> It all sounds a bit odd to me, I think it's more likely that the meter
>> that says the voltage is 280 volts is mis-reading for some reason.
>> Does anyone here have any knowledge of such things?
>>
>> (We don't currently know the manufacturer, nor has the '110 volt'
>> output been measured)
>
> My /guess/ is that at very low secondary currents - almost none really -
> the transformer core hysteresis is significant. As the very small
> primary current changes polarity, there's a period when it's not
> magnetising the core, then the hysteresis is overcome, so the magnetic
> field 'jumps' as it changes direction.

Most small transformers have poor regulation. 10-20% is not to be
unexpected. 280V still sounds high but without knowing the spec and
working practice associated with these shavers sockets it is difficult
to comment.

The field is driven by magnetising current but the magnitude of the
field is dependent on the applied voltage.

Any 'jump' in field will therefore be effectively clamped by the applied
AC mains voltage. Any field leakage will also be of the same order of
10-20% and so any spike or non-sinusoidal element should also be limited
to 10-20%.

> This will affect the wave shape which may affect the measured secondary
> voltage and will affect the actual voltage howsoever defined in some way.

Cheap DVMs tend to fudge rms measurement of AC voltages and make
assumptions about the waveform. I might suggest 'spikes' could have a
disproportionate effect on a measurement.

> Basically, no load isn't a fair test.

Agreed.

[Dave Plowman (News)]

In article <lg1cli-6...@esprimo.zbmc.eu>,

Open circuit voltage measurements with a transformer are pretty pointless.
Measure the voltage under load.

--
*I was once a millionaire but my mom gave away my baseball cards

Dave Plowman da...@davenoise.co.uk London SW
To e-mail, change noise into sound.

[charles]

In article <ushcli-q...@esprimo.zbmc.eu>, Chris Green <c...@isbd.net>
wrote:

checking here,I find the shaver socket shows 270v ac while an ordinary
socket shows 241v on the same meter,

--
from KT24 in Surrey, England
"I'd rather die of exhaustion than die of boredom" Thomas Carlyle

[Dave Plowman (News)]

In article <59ea04f2...@candehope.me.uk>,

Not quite sure how design influences it, but every transformer I've come
across gives more volts off load than on. And a low cost DVM could also
have quite a high tollenace on AC. Assuming even the waveform is good.

--
*If vegetable oil comes from vegetables, where does baby oil come from? *

[williamwright]

On 17/05/2022 14:20, Dave Plowman (News) wrote:
> Open circuit voltage measurements with a transformer are pretty pointless.
> Measure the voltage under load.

We look after a large communal TV system that works using 44VAC line
power. 4:1 transformers produce the nominal 44V from the mains supply.
With no load the output is about 55V (which is not a quarter of mains
voltage!). With a normal load the line voltage is about 44V. At the
repeaters we use 5:1 transformers to step the power up to about 220V
when there's a load. That's so we can use mains equipment.

It all works pretty well; keeps slogging on! It is virtually immune to
EMP from lightning, unlike DC systems, which get splatted every time.

Bill

[Chris Green]

So what BIL is seeing isn't unusual/unreasonable, you learn something
every day! :-)

--
Chris Green
·

[Paul]

https://professional-electrician.com/technical/socket-outlets-shavers/

"Neither the rated supply voltage nor rated output voltage of a
shaver supply may exceed 250 V a.c. The no-load output voltage is
not to exceed 275 V and the difference between the no-load output voltage
and the output voltage under load shall not exceed 20 % of the
load output voltage.

For example, a no-load voltage of 275 V and an output load voltage
of 230 V would lead to percentage variance approaching 20 %. The output
voltamperes must be not less than 20 VA and not more than 50 VA.

The transformer of a shaver supply unit is an isolating transformer,
intended to provide protection against electric shock by ‘electrical separation’.

The transformer is of the short-circuit proof type (either inherently or noninherently).

A thermal cut-out is provided in a shaver supply unit to limit the
temperature of the transformer under conditions of short-circuit or overload.

The cut-out may be of either the self-resetting type or the manually resetting type.
"

In other words, a box of squirrels rather than a regular take-no-prisoners transformer.
They may have played with the Core Saturation characteristic or something.

https://www.allaboutcircuits.com/textbook/alternating-current/chpt-9/practical-considerations-transformers/

Core Saturation

Examination with a battery-operated oscilloscope may indicate
just how screwed-up the waveform is. Normally, you would not
want harmonic content in the output waveform, because other
motors or transformers would not like that. Who knows what
the shaver looks like as a load... Maybe another box of
squirrels is involved.

Paul

[John Rumm]

They are designed for loads between 20 and 50 VA - so not huge loads.