Should it be possible to receive an electric shock from a FireWire cable?

[Miss Perspicacia Tick]

I posted here the other day after having problems with a Maxtor external
drive. Well, after much faffing around, I got that sorted, and I booted to
Windows (XP SP2). Then the fun *really* began. For starters it insisted the
drive contained pictures (when it had been thrice formatted) and nothing I
tried could persuade Windows otherwise - then it continually caused my
system to lock up (it's externally powered it should *NOT* cause any issues
with the system - besides, I have a 500W Tagan PSU, not exactly pound
shop...) now I've removed it, it's like the whole rig's breathed a huge sigh
of relief - it's going back - I will *never* purchase another Crapstor
product as long as I draw breath!

Anyway, as I pulled the FireWire cable, I accidentally touched the end - and
immediately dropped it. Whilst the shock wasn't severe, it did burn my
finger. Yes, I will admit the drive was still plugged in and it was
receiving power, but I have since (yes, I'm a masochist) tried the same
thing with my other FireWire devices, namely my scanner, printers and card
reader and my fingers are fine.

Is this usual with external drives, or is it a sign the unit wasn't properly
earthed?

 

[Noozer]

Anyway, as I pulled the FireWire cable, I accidentally touched the end - and
immediately dropped it. Whilst the shock wasn't severe, it did burn my
finger. Yes, I will admit the drive was still plugged in and it was
receiving power, but I have since (yes, I'm a masochist) tried the same
thing with my other FireWire devices, namely my scanner, printers and card
reader and my fingers are fine.

You should definately NOT get zapped by the end of a firewire or USB plug...

 

[Gerard Bok]

You should definately NOT get zapped by the end of a firewire or USB plug...

You will, however, if you fail to provide proper grounding for
your PC!

Use a properly connected 3-pin socket and a 3 pin plug.
If you still get a shock, call in a certified electrician and
have the socket checked !

Faulty wiring can be lethal. And may void your insurance.

 

[Michael C]

You will, however, if you fail to provide proper grounding for
your PC!

Use a properly connected 3-pin socket and a 3 pin plug.
If you still get a shock, call in a certified electrician and
have the socket checked !

Faulty wiring can be lethal. And may void your insurance.

It is common for scanners and other peripherials to spark when plugged into
a PC. They can sit at a higher voltage because they only have 2 pin power
plugs so I guess this higher voltage could shock you. Interestingly it's
actually the earth wire of the firewire/usb cable that shocks you, not the
power or data pins.

Michael

 

[Gerard Bok]

It is common for scanners and other peripherials to spark when plugged into
a PC.

Well, common as in 'it happens to several people'.
Not common as in 'this is normal behavour'.

Any electronic equipment requires some kind of noise filter on
it's mains input.
And all such filters operate in the same manner.
Basically: they 'collect' the noise (both incoming and outgoing)
and route it to the safety-ground. (The 3rd pin on your plug.)

Result of this design is --always-- that with an unconnected
safety ground, any metal parts of your PC --and connected
peripherals-- reside at half your mains voltage.

In the USA, this means some 55 volt. And thats the upper limit
for safe operation. (And yes, that is exactly the reason why they
desided on 110 Volt and not on, say 200 volts.)

In Europe, this means a case would float at some 120 Volts. That
is dangerous and therefore illegal.

Interestingly it's actually the earth wire of the firewire/usb
cable that shocks you, not the power or data pins.

Well observed. And explained above

 

[Michael C]

Well, common as in 'it happens to several people'.
Not common as in 'this is normal behavour'.

It would happen with millions of peripherals so it is fairly common.

Any electronic equipment requires some kind of noise filter on
it's mains input.
And all such filters operate in the same manner.
Basically: they 'collect' the noise (both incoming and outgoing)
and route it to the safety-ground. (The 3rd pin on your plug.)

But many don't have a third pin. I'll measure the voltage on the case of my
scanner tonight when it is unplugged from the PC.

Michael

 

[Gerard Bok]

It would happen with millions of peripherals so it is fairly common.

You misunderstood. Or I did not make it clear enough.

But many don't have a third pin.

I very much doubt if you can find any desktop PC that does not
have a 3-pin mains connector

I'll measure the voltage on the case of my
scanner tonight when it is unplugged from the PC.

That won't tell you much. Your scanner is likely to have a
transformer. Hence no measurable voltage on any metal part.

A peripheral connector that sparks does so, because it carries
some current from the PC's filter that should have flown through
the safety-pin.

Make a proper 3-wire connector for the PC and you won't see any
sparks, nor any blown ports, nor feel any shock.
(Well, apart from static from your carpet, maybe

 

[Michael C]

You misunderstood. Or I did not make it clear enough.

You didn't make yourself clear.

I very much doubt if you can find any desktop PC that does not
have a 3-pin mains connector

It's the peripherals that have the 2 pin connector which allows their case
to float at a voltage. Because the PC is earthed current flows to the PC.

That won't tell you much. Your scanner is likely to have a
transformer. Hence no measurable voltage on any metal part.

It certainly has enough to spark so I'd be suprised if I couldn't measure
it.

A peripheral connector that sparks does so, because it carries
some current from the PC's filter that should have flown through
the safety-pin.

No, it sparks because the peripheral's case has a voltage and the PC earths
that out.

Make a proper 3-wire connector for the PC and you won't see any
sparks, nor any blown ports, nor feel any shock.
(Well, apart from static from your carpet, maybe

My PC has a three pin power connector and it sparks and can give a small
shock when the scanner is plugged in or out.

Michael

 

[Gerard Bok]

You didn't make yourself clear.


It's the peripherals that have the 2 pin connector which allows their case
to float at a voltage. Because the PC is earthed current flows to the PC.

I must admit, that this looks like a plausible explanation. But
yet, it is wrong.
2-pin appliances are fully insulated and do not allow any current
to flow between mains and any metal part. (Unless they are
broken, that is.)

It is 3-pin appliances that allow for some small capacitors
between mains and earth. That's all right as long as the
appliance is properly connected to safety ground.
And dangarous if they aren't.

It certainly has enough to spark so I'd be suprised if I couldn't measure
it.

If you can measure any voltage between earth and a 2-sprong
appliance, the appliance is faulty.
(And don't be so sure about 'being able to measure'. If you comb
your hair you may see sparks in the dark. But you won't be able
to measure them with a multimeter

No, it sparks because the peripheral's case has a voltage and the PC earths
that out.

Again, it makes sense. But the peripheral's case cannot carry
any current unless the device is faulty.

 

[Michael C]

If you can measure any voltage between earth and a 2-sprong
appliance, the appliance is faulty.

There must be a lot of faulty appliances out there because this is very
common. It is the reason that it is recommended not to plug a printer into a
PC when it is running. I think you'll find that peripherals with a 2 pin
plug shouldn't have a case voltage but very often come out of reputable
factories that way.

BTW, I forgot to measure the voltage, I'll have to do it tonight.
Michael