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Topic Title: weird fault with laptop
Topic Summary: Shocks from laptop
Created On: 15 December 2010 06:14 PM
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 15 December 2010 06:14 PM
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ArthurHall

Posts: 735
Joined: 25 July 2008

I would appriciate some help
I have a Dell D531 which I use mainly in substations, so lots of bonding and every thing well earthed.
Recently I have been getting shocks from exposed metal parts of the laptop. I have measured between 90 and 120 volts between the laptop and earth. The voltage is constant so not due to static. If I unplug the PSU and the USB cable to the printer the voltage goes down to about 5 volts, pug either in again and its back to about a 100 volts.
The laptop works fine, I have tried shorting the laptop to earth but as soon as the earth is removed the voltage goes back up.
The current substation I am working in has not been connected so no HV in the area.
Any ideas?
 16 December 2010 11:39 AM
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ectophile

Posts: 539
Joined: 17 September 2001

The two most likely possibilities I can see are:

1. A fault with the laptop's power supply. The DC output should either be earthed, or completely isolated from the incoming AC, depending on the design. If this isn't the case, then AC could be leaking through to the laptop.

2. The DC output of the power supply is supposed to be earthed, but you have a broken earth somewhere. Perhaps the problem is in the mains lead, or in the fixed wiring to the socket.

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S P Barker BSc PhD MIET
 20 December 2010 07:48 AM
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dickllewellyn

Posts: 1150
Joined: 19 March 2010

My girlfriends laptop apears to make herr "live"! When she has it on her lap, you can touch her and feel the fizzing of a touch voltage. If I sit next to her on the sofa with a volt stick in my pocket, it instantly starts to buzz etc.

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Regards
Richard (Dick)

"Insert words of wisdom and/or witty pun here"
 20 December 2010 05:56 PM
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ArthurHall

Posts: 735
Joined: 25 July 2008

Simon, thanks for your thoughts. The power lead from the transformer unit to the laptop is 2 core as is the lead from the plug top to the transformer, so there is no proper earth connection although there may be via a capacitor.
The problem is worst when my work printer is used so I am starting to think the problem is with the printer and the charge is transfered via the USB lead.
I will have a look at the printer.
The 240v socket is new, steel conduit with an eath wire as well.

Edited: 21 December 2010 at 07:42 AM by ArthurHall
 21 December 2010 05:49 PM
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AlanKay

Posts: 232
Joined: 09 July 2002

Sounds like a switch-mode PSU with interference-suppression capacitors from incoming phase and neutral to the PSU's secondary earth. The caps create a current-limited potential divider - so you see half 240V to earth.
I've seen the same problem with Sony laptop PSUs.
They must have passed some sort of acceptance testing to be sold in the UK wouldn't you think?...

Alan

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Alan Kay, CEng MIEE
 23 December 2010 08:47 AM
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ectophile

Posts: 539
Joined: 17 September 2001

USB leads are normally screened, with the metal bodies of the two plugs connected to each other. So if the printer chassis is at 120V, then it's will raise the laptop chassis to 120V as soon as they are connected.

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S P Barker BSc PhD MIET
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