OT - Double-insulted equipment safety query

Sorry for posting this here, but alt.comp.hardware produced no response, and
I know that the regulars here are technically inclined and might have a
clue.

I recently bought an external 320GB HDD, the power switch for which is at
the back. A few times, when reaching around the back, the back of my hand
touched another device and I received a slight electric shock. On
investigating I found that there was about 80V AC to earth on the edge of
the metal case. The HDD is connected to a Linksys NSLU2 NSA. When this is
switched off, the voltage on the HDD drops to about 40V AC.

I found that both PSUs had this voltage on the outer connector of their
power lead. Neither psu has a ground connection. A replacement psu from
Iomega has the same feature.

As the HDD has no earth, but does have a metal case, what are the safety
requirements, if any, in the UK?

Allan

allan wrote:

> Sorry for posting this here, but alt.comp.hardware produced no response, and
> I know that the regulars here are technically inclined and might have a
> clue.
>
> I recently bought an external 320GB HDD, the power switch for which is at
> the back. A few times, when reaching around the back, the back of my hand
> touched another device and I received a slight electric shock. On
> investigating I found that there was about 80V AC to earth on the edge of
> the metal case. The HDD is connected to a Linksys NSLU2 NSA. When this is
> switched off, the voltage on the HDD drops to about 40V AC.
>
> I found that both PSUs had this voltage on the outer connector of their
> power lead. Neither psu has a ground connection. A replacement psu from
> Iomega has the same feature.
>
> As the HDD has no earth, but does have a metal case, what are the safety
> requirements, if any, in the UK?

Believe it or not, a small leakage current is compliant with EN60950. I could
even tell you which component's responsible ! It's called a Y capacitor and it
bridges the primary and secondary in your power supplies. Its value is likely in
the region of 470 - 2200 pF.

I have a copy of EN60065 here (virtually identical to EN60950) and if I get a
moment I'll check the numbers for Class 2 equipment.

Graham

On 26-Apr-2007, Eeyore <(E-Mail Removed)> wrote:

> Neither psu has a ground connection. A replacement psu from
> > Iomega has the same feature.

Having a rwo wire mains connection to a product with a metal
case is dubious practice. EN6950 might permit a Y capacitor,
but what if it breaks down to case?
Is 100% reliability, even allowing for spikes and condensation,
a feature of Y capacitors?

<(E-Mail Removed)> wrote in message
news:(E-Mail Removed)...
>
> On 26-Apr-2007, Eeyore <(E-Mail Removed)> wrote:
>
>> Neither psu has a ground connection. A replacement psu from
>> > Iomega has the same feature.
>
> Having a rwo wire mains connection to a product with a metal
> case is dubious practice. EN6950 might permit a Y capacitor,
> but what if it breaks down to case?
> Is 100% reliability, even allowing for spikes and condensation,
> a feature of Y capacitors?

Most switch-mode power supplies will have a capacitor from live to earth and
another from neutral to earth as part of their RFI suppression. It follows
that a small earth current will flow, which gives problems when 50 or so
such devices are connected to a single eath leakage circuit breaker rated at
20mA.

And of course, if the case of the device is metal and it has no earth
connection, you will feel a mild electric shock if you touch it.

--
Graham J (no connction with anybody else on this ng called Graham)

On 27-Apr-2007, "Graham" <(E-Mail Removed)> wrote:

> Most switch-mode power supplies will have a capacitor from live to earth and
> another from neutral to earth as part of their RFI suppression. It follows
> that a small earth current will flow, which gives problems when 50 or so
> such devices are connected to a single eath leakage circuit breaker rated at
> 20mA.
>
> And of course, if the case of the device is metal and it has no earth
> connection, you will feel a mild electric shock if you touch it.

And what happens if the Y capacitor goes short circuit to the metal
case, and there is no earth leakage current breaker?

The OP said it was a 2-wire fed backup drive with a metal case, so
it would become live in relation to the 2-wire plus earth, grounded
PC case.

Potentially leathal I'd say.

(E-Mail Removed) wrote in message
(E-Mail Removed):

> On 27-Apr-2007, "Graham" <(E-Mail Removed)> wrote:
>
>> Most switch-mode power supplies will have a capacitor from live to
>> earth and another from neutral to earth as part of their RFI
>> suppression. It follows that a small earth current will flow, which
>> gives problems when 50 or so such devices are connected to a single
>> eath leakage circuit breaker rated at 20mA.
>>
>> And of course, if the case of the device is metal and it has no earth
>> connection, you will feel a mild electric shock if you touch it.
>
> And what happens if the Y capacitor goes short circuit to the metal
> case, and there is no earth leakage current breaker?
>
> The OP said it was a 2-wire fed backup drive with a metal case, so
> it would become live in relation to the 2-wire plus earth, grounded
> PC case.
>
> Potentially leathal I'd say.

My TV (a 7-year-old Panasonic) presents about 200 V AC between the screen of
the aerial lead and ground, although this falls to about 50 V when a
human-size load (say around 500 kilohms) is applied. This is definitely
strong enought to feel.

I first discovered it when I was plugging my aerial lead into my computer's
video card. The computer is earthed. I had one hand on the lead and the
other on the case of the computer and I felt a strong tingle. I wouldn't
want to experience it for more than a few seconds. Panasonic said that it
was within normal safety limits.

I've now wrapped the metal aerial plug in insulation tape so I don't get a
shock when I plug it in.

(E-Mail Removed) wrote:
>
> On 27-Apr-2007, "Graham" <(E-Mail Removed)> wrote:
>
> > Most switch-mode power supplies will have a capacitor from live to earth and
> > another from neutral to earth as part of their RFI suppression. It follows
> > that a small earth current will flow, which gives problems when 50 or so
> > such devices are connected to a single eath leakage circuit breaker rated at
> > 20mA.
> >
> > And of course, if the case of the device is metal and it has no earth
> > connection, you will feel a mild electric shock if you touch it.
>
> And what happens if the Y capacitor goes short circuit to the metal
> case, and there is no earth leakage current breaker?
>
> The OP said it was a 2-wire fed backup drive with a metal case, so
> it would become live in relation to the 2-wire plus earth, grounded
> PC case.
>
> Potentially leathal I'd say.

"Double Insulated" (if it is) means there are stringent requirements
on the layout etc. inside the device, these should make pretty certain
that a capacitor short won't be to the case.

In addition there are also stringent safety rules for the capacitors
used for this application such that they should fail safe.

Whether this particular device conforms to all these regulations is,
of course, not certain.

--
Chris Green

(E-Mail Removed) wrote:

> Eeyore <(E-Mail Removed)> wrote:
>
> > Neither psu has a ground connection. A replacement psu from
> > > Iomega has the same feature.
>
> Having a rwo wire mains connection to a product with a metal
> case is dubious practice.

Really ? It's amazingly common these days. Do you have a set-top box for example
? Or a hi-fi ? Many countries outside the UK have only nominal earthing
available !


> EN6950 might permit a Y capacitor,but what if it breaks down to case?
> Is 100% reliability, even allowing for spikes and condensation,
> a feature of Y capacitors?

Yes.

It's down to the construction and test voltage (4kV?). I should have been more
precise actually. The Y cap allowed in Class 2 equipment has to be of the type
that internally consists of 2 capacitors in series. Y4 types certainly meet that
spec. I don't think Y1 types do, you'd physically have to use 2 of those in
series, so there's no 'single point failure' mode.

Graham

(E-Mail Removed) wrote:

> And what happens if the Y capacitor goes short circuit

They don't. It's a safety type construction.


> to the metal case, and there is no earth leakage current breaker?
>
> The OP said it was a 2-wire fed backup drive with a metal case, so
> it would become live in relation to the 2-wire plus earth, grounded
> PC case.
>
> Potentially leathal I'd say.

No. An amazing amount of what you buy these days has one.

The Y cap in the OP's PSU actually 'bridges' the high-frequncy transformer's
primary and secondary circuits.

Graham

(E-Mail Removed) wrote:

> "Double Insulated" (if it is) means there are stringent requirements
> on the layout etc. inside the device, these should make pretty certain
> that a capacitor short won't be to the case.

It could very well be connected to case actually.

Y4 caps don't short though. They are usually emblazoned with safety agency logos.

Graham