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How Should I Proactively check for Potential Shock Hazards?

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  • Mick Bailey
    replied
    Originally posted by Guitarist1983 View Post
    .....the 3-prong (USA) multi-outlet extension cord I plugged into was missing the ground plug and it's hot & common blades were inserted backwards into the wall.
    The effect of a missing mains earth has already been covered. L-N reversal provides another reason for the presence of a high voltage with certain SMPS designs; there can be a capacitor connected between the primary and secondary of the transformer which connects the 0v (DC ground) output to the neutral side of the mains supply. When the mains connection is reversed it instead connects to the live side and causes a high voltage to appear on the output ground. Again, this is a high impedance source, though with everything earthed in itself does not give rise to a shock situation except under certain conditions. It isn't however something to ignore - it's still a fault.

    An example of this is I investigated a situation where a guitarist/singer was getting shocked off his microphone if his lips touched it. He bought along his complete setup and I measured something like 80v between his mic and ground. He was using a TC Helicon harmonizer pedal with a genuine PSU of the wall-wart type. In the UK we have shuttered mains sockets where the longer earth pin opens the shutters for the L&N. The PSU has a moulded dummy plastic earth pin which had broken off and a bandmate had advised that the singer could use the PSU if he used a screwdriver in the socket earth to lever open the socket shutter and then plug in the PSU upside down. This of course reversed the mains connection and was causng the shock due to the ground becoming 'hot' due the capacitor connection. I connected the PSU correctly and it immediately resolved the shock issue. All he needed to do was buy another PSU.

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  • Guitarist1983
    replied
    Originally posted by pdf64 View Post
    I don’t see how that would make your pedalboard chassis hot, unless it was mis-wired, eg mains inlet neutral terminal connected to the chassis metalwork. ?
    Now the pedalboard's audio output was connected to a Radial SGI powered from a correctly configured mains outlet and then sent on to the amp powered from another correctly configured mains outlet. Would this cause issues with the hot/neutral mismatches among the pedalboard's patch cables and amp? (It should be obvious by now that I'm not very knowledgeable about electricity). I think I'll get the multiplug extension cord out of the trash and check it for ground/common funkiness.
    Last edited by Guitarist1983; 04-22-2021, 02:51 AM.

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  • pdf64
    replied
    Originally posted by Guitarist1983 View Post
    Update: I went back to the venue to test the outlets. Bingo, the 3-prong (USA) multi-outlet extension cord I plugged into was missing the ground plug and it's hot & common blades were inserted backwards into the wall. Hence my pedalboard chassis was hot...
    I don’t see how that would make your pedalboard chassis hot, unless it was mis-wired, eg mains inlet neutral terminal connected to the chassis metalwork. ?

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  • Guitarist1983
    replied
    Update: I went back to the venue to test the outlets. Bingo, the 3-prong (USA) multi-outlet extension cord I plugged into was missing the ground plug and it's hot & common blades were inserted backwards into the wall. Hence my pedalboard chassis was hot. That cord is now in the dust bin. I now have an outlet tester that will go in my cable bag.

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  • Mick Bailey
    replied
    Apart from the SMPS issue, you can improve your own safety by using a plug-in mains tester to determine if there's a wiring problem either with your equipment (extension leads) or with the venue's supply. Common faults are L-N reversal and disconnected mains earth. Buy a good quality certified unit. Another thing to consider is using your own earth leakage device - either a plug-in type or one built into an extension lead. I have a metal-clad MK socket hard wired on the end of a foot of cable that I used to carry round with me.

    The problem is what to do if you're setting up and find there's a problem with the venue supply. I have one customer who uses an isolation transformer along with a voltage conditioner because he's so fed up with venues that have supply issues.
    Last edited by Mick Bailey; 04-21-2021, 04:07 PM.

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  • Helmholtz
    replied
    I have that SMPS problem with the power supply of my EPSON printer. With USB cable connected to my notebook I got shocked when touching metal parts.
    I measured the voltage from the USB plug to safety earth with a 10M meter and found around 100V.

    As remedy I safety grounded the chassis of my docking station using a separate mains plug.

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  • pdf64
    replied
    Yes, I’ve noticed that with SMPS too - somewhat off putting at first.
    And also thereafter...

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  • Mick Bailey
    replied
    After eliminating any possibility of a disconnected equipment mains earth, or other fault, it may be down being a switch mode power supply (SMPS). There can be around 60v-80v between the DC output and mains earth with some supplies. If you measure the DC output of the PSU, you'll read 9v, but if you measure from the DC plug to something connected to mains earth - a water pipe or central heating radiator - you'll see a much higher voltage. With the equipment connected and your amp earthed, this ghost voltage gets grounded and reduces to near-zero because the circuit is high impedance. There should be no shock with everything connected. If this is still the case, make sure your amp is properly earthed and if you're using an extension lead, check that as well.

    One thing about using a transformer balanced coupling to your amp is it probably breaks the earth connection to the amp. Try your setup without the transformer, or make sure the ground is not lifted.

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  • Helmholtz
    replied
    Get a mains tester (screwdriver with built in neon bulb).

    I suppose a failed safety ground connection in mains connected equipment or house installation.

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  • How Should I Proactively check for Potential Shock Hazards?

    During a recent sound check, I touched my pedal board and received a mild shock. I got shocked by the pedal board chassis, and in some cases the Tip-Sleeve patch cables. Because I was limited in time, I powered down the board (VooDoo Lab Power Supplies) and pulled off all pedals and cables I didn't need to use that day. No more shock. Later I inspected the cables and could find no damage or exposed wire (though I still think this must be the issue--but what do I know?)

    Anyway, the real questions?
    How should I proactively check my pedalboard, etc in the future for shock hazards without using my fingers?
    How would you test to find the source of the problem? How in the world did the pedalboard itself go "hot"?
    Also, can 9v DC power from a VooDoo Lab PS (400mA rated) create a shock hazard?

    Other info: Touching the guitar strings did not shock me; on a dry, wooden stage; I was touching nothing else when I got shocked, All outlets and A/C plugs were 3-prong, building is 12-years old. USA.
    Signal path: Guitar to typical 9v DC pedal chain powered by 2 VooDoo Lab PSs, Audio output from pedalboard to Radial impedence transformer balanced line to amp 75' away.
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