Voltage on DVD case??

Hi,

Is it usual to have a voltage of 100+ V AC present on a case screw of a DVD player? I've measured the voltage between the case screw and a known earth (in my case the earth pin of a nearby wall socket) using a multimeter.

The DVD player is double insulated. I'm currently in a business premises with 3 phase. Both the earth pin I used to test and the DVD player are on the same phase.

I found it when I noticed a tingling on the back of my arm when connecting some AV leads and lent on the case slightly. I've tested another DVD player and have voltage on that too! Is this right?

Other equipment I've tested, like the TV, kettle and phone system, don't have a voltage present on any of the case screws.

Should I worry????

Many thanks

robJ71 said:

Is it usual to have a voltage of 100+ V AC present on a case screw of a DVD player? I've measured the voltage between the case screw and a known earth (in my case the earth pin of a nearby wall socket) using a multimeter. ... The DVD player is double insulated.

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If it's double insulated, the case/screw will be 'floating' electrically, and is therefore quite likely to end up with a potential relative to earth of roughly half the supply voltage (i.e. around 115V with a 230V supply) - due to 'leakage', most commonly due to filter capacitors. However, the current-supplying capacity of that potential will be extremely low (far too low to 'do harm') - and you're probably only seeing it because you are using a high impedance multimeter which does not draw enough current to 'kill' that voltage. If you put a resistor (or other load) across the multimeter, then the voltage would probably almost 'disappear'.

So, yes, it's not usual. However, it's obviously not totally impossible that, even with double insulation, a fault could develop which made the case genuinely (and dangerously) live. However, if all you've noticed when you touched it (and earth) was a tingling (rather than a major shock), the the above explanation is very probably the explanation.

Kind Regards, John

If the equipment has only Live and Neutral with an Earth in the cable then the metal work of the case although double insulated will have a potential of around 115 volts but with a very high source impedance so no significant current can be taken.

The voltage is due to capacitive coupling between case and Live and case and Neutral forming a voltage divider.

Although the current that can be taken from the case is limited by the amount of capacitive coupling so as to not present a serious shock hazard it can be enough to be an irritating tingle.

( too slow typing )

Thanks Guys, that puts my mind at rest.

Thanks for the quick and comprehensive explanations.

Class 2 appliances have no protective earth to return leakage currents to so they end up as "touch currents". For this reason the leakage currents allowed for for class 2 appliances are much lower than for class 1 appliances and are too low for most people to feel.

It can also become worse when you connect together multiple items of double insulated equipment using non-isolated interfaces effectively summing up the touch currents of the appliances. Theres a reason communal sattelite TV systems are tied to the block's protective earth.

The question is are you just unusually sensitive, is there a fault with either the DVD player or another connected appliance or have you connected too much class 2 equipment together using non-isolated interfaces?

From a quick googling it seems that for class 2 appliances this current should be below 0.25ma. Personally i'd set my multimeter to current mode, connect it between the case and earth and see how much current flows.

plugwash said:

Personally i'd set my multimeter to current mode, connect it between the case and earth and see how much current flows.

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It would be nice to be able to do that, but I don't think that (m)any 'everyday' (aka cheap!) multimeters have the ability to measure AC current. For those who have access to resistors, putting, say, a 100 kΩ resistor (which would carry 1 mA if 100V were put across it) across the multimeter should result is a very substantial fall in the ~100V voltage being measured. Decent meters often have the ability to switch a 'shunt' resistor across its input when measuring voltage.

Of course, if you have a multimeter that can measure AC current and did as you suggest, and if it happened that there were a 'real fault' (albeit very unlikely), you might blow up the meter, if it's not well fused

Kind Regards, John

JohnW2 said:

Of course, if you have a multimeter that can measure AC current and did as you suggest, and if it happened that there were a 'real fault' (albeit very unlikely), you might blow up the meter, if it's not well fused

Click to expand...

And it'd deserve it.