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If there is electric current on a material surface (but not an obvious thing like outlet hole or such), what is the best tool to check it ? Is it possible to use multimeter?

Example case:
There is a new laptop I bought, once I connected the charger to AC current for charging the laptop, I noticed that wherever I touch any metal part on the laptop or certain surface area, I would get electric Shock. If I plug off the charger, there is no issue at all.

Assumed the issue is in the laptop, not the house's grounding or any, I want to return the laptop, but I need to show a proof to the laptop seller that the laptop has electric shock issue. In this case, what is the best tool to show there is electric current on the laptop ?

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  • \$\begingroup\$ What are you wearing? Seriously, could be static discharge. \$\endgroup\$ – Eugene Sh. Sep 29 '17 at 13:56
  • \$\begingroup\$ Assumed I wear nothing that could lead a static discharge. \$\endgroup\$ – null Sep 29 '17 at 13:56
  • \$\begingroup\$ If it is consistent you can just prove it by plugging it into the wall at the service center and letting the technician to touch it... \$\endgroup\$ – Eugene Sh. Sep 29 '17 at 13:57
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    \$\begingroup\$ This is not uncommon with isolated power supplies. Cuplrit is usually the capacitors that connect between mains and DC. \$\endgroup\$ – Joren Vaes Sep 29 '17 at 13:57
  • \$\begingroup\$ You can't just use a multimeter because the laptop is floating. You need a low-z mode at least to prevent ghost voltages due to capacitive coupleing \$\endgroup\$ – Joren Vaes Sep 29 '17 at 13:58
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Try using this (https://en.wikipedia.org/wiki/Test_light#One-contact_neon_test_lights). We call it a test pen here. They are cheap, and simple enough to understand - lights up = death. Your customer service guy cannot dispute your claim if he sees this.

For greater sensitivity use while bare footed:)

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  • \$\begingroup\$ So you mean if the test pen lights up, the test pen will die and becomes unusable? \$\endgroup\$ – null Sep 29 '17 at 14:52
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    \$\begingroup\$ Nope. If the test pen lights up when it touches an object, it means that a high voltage is present, and thus not safe to touch. Electricians use this tool to check for live wires and leakages before they start to work on something. Remember, your bare finger must touch the metal piece at the end of the test pen's handle for it to work. \$\endgroup\$ – ckchai Sep 30 '17 at 2:07
  • \$\begingroup\$ I found there is non-contact tester pen, for my example case above, which is better between normal/direct-contact tester pen and non-contact tester pen ? \$\endgroup\$ – null Sep 30 '17 at 2:15
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    \$\begingroup\$ Such a tester will not have the same impedance as a safety-test would require (there are multiple different testing standards, each with a load impedance specification). It's good for a quick check, but not as accurate as, for instance, a ground-fault interrupter. \$\endgroup\$ – Whit3rd Sep 30 '17 at 10:54
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    \$\begingroup\$ Can't comment on the non contact ones really, I never own one. I had always used the contact type one - they are dirt cheap and doubles as a screwdriver. And yup, they are for general use only, not for any kind of standardized testing. \$\endgroup\$ – ckchai Sep 30 '17 at 13:50
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The surface has a common mode potential from the AC line filter and the SMPS transformer which both contribute to surface voltage relative to earth ground. The current is not through the surface until you are connected between earth ground and the metal case. THese currents are minimal yet exist for EMC reasons and for safety reasons, left floating in case of ground faults and you get between them.

There are two paths that connect you to this surface potential to earth ground.

1) Y caps on CM choke to earth pin in charger are limited by safety design to 250uA at line f.

2) transformer is floating between AC and DC out yet there is still leakage from stray capacitance primary to secondary now at SMPS rate because the wiring is not balanced like a CM choke and DC CM chokes are large and expensive, so they exclude them and just add a small CM choke in the form of a Ferrite torroid or clam-shell ferrite molded around the DC cable.

Hence a small skin surface area in MOhms range ( if measured on a DMM) is high compared to the Zc(f) of your skin contact with a large dielectric behind it (you) and a large surface area ( your feet with sweaty socks and leather shoes on non-dry concrete or other hand touching earth ground ) Thus the impedance of the stray coupling capacitance and primary voltage is the path which tends to have a burning sensation more than the Y cap tingle at line f.

Both are safety tested for Y cap leakage , but not always correctly tested with secondary earth ground currents measured.

I too have a concern with this.

Yet designers are aware this must be avoided to prevent these RF burns from switched pulses with fast risetimes and the stray series capacitance is chosen to limit this annoyance sensation which is only serious in medical applications.

If someone has specific safety standards to test unbalanced RF leakage currents via transformer coupling capacitance, you may add to this.

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  • \$\begingroup\$ I'm glad to see you write about this. I'm also concerned about a recent trend here. For example, my Microsoft Surface Book has an easily measurable AC voltage on its metallic surface (even with a relatively low impedance load on it.) And I can feel the shock, as well. A fix is that I reverse the plug in the socket, which removes this problem the moment I do it. I contacted Microsoft, gave them my measurements and circumstances, and after a month they told me "it's normal." +1 \$\endgroup\$ – jonk Sep 29 '17 at 17:31
  • \$\begingroup\$ I tried to measure it today and found my old HP 6 digit multimeter "out of calibration" oh well, diff probes and a shunt on a scope will do it. Yes the line leakage capacitance on 3 pin chargers is well controlled but for not a 2 pin plug. It is mainly in the finger capacitance ratio, current density and 2nd gnd path that causes us to react with transformer coupling stray winding C for line vs neutral orientation. Plugs are not keyed for this convenience and creates anx for some who dont understand and never warned. \$\endgroup\$ – Sunnyskyguy EE75 Sep 30 '17 at 0:09
  • \$\begingroup\$ The 2 pronged SMPS regulators are supposed to be better due to the isolation from line EMI ingress and egress, yet can be worse for transformer interwinding capacitance and hence primary switched high freq, high voltage coupled to the output as a common mode linked more to line or neutral. I suspect the current is still limited ut the sensation is different and a tiny case touch on the kneecap can feel like a needle instead of a tingle. \$\endgroup\$ – Sunnyskyguy EE75 Sep 30 '17 at 0:23

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