I think our company fields machines that can shock people. Can I try to fix it?

Question

_{For more background details, you should read my original question on Workplace Stack Exchange.}

I am a technician at an arcade. My company is fielding machines that seem to be shocking people from the outside. My question is "What could be causing this?"

People get shocked if they touch metal surfaces on the outside of the machine (such as locks!), so I was wondering:

• Can this be caused by faulty wiring leading to the machines?
• If this is truly a faulty ground, why is the ground carrying current at all? I thought the ground was only for if something went wrong.
• I got shocked by a faceplate underneath a joystick my first day, and my boss fixed it by cutting a "ground wire" from the panel. How horrible is this? (It's happened more than once, in widely separated areas.).

Perhaps most importantly, do any tools exist so that I could check for a consistent potential on the face of the machines? (Wetting the local area then shocking myself while I'm wearing socks is not my idea of a great test.) This way I can at least definitively prove a problem exists; see my original question for why this might be difficult.

Note: I know about voltmeters, but I'm wondering if there exists something more specialized; normal voltmeters would be difficult to use with the geometry of the environment. (I'd need very long probes)

Answer 1

If the shocks are large and muscle grabbing then the explanation below is NOT the cause and the source should be sought with care. However, there is a reasonable chance that the cause is as follows.

IF the equipment has NOT got a ground lead and/or grounded 3rd pin in the mains power socket then noise filter caps in the equipment (connected from both power leads to equipment ground) can cause the equipment ground to be at about half mains AC voltage BUT at high impedance.

The shocks received from this source are annoying but probably not dangerous.

In the left hand image below the Cx and Cy noise filter capacitors connect to the metal case which is connected to mains ground (PE).

If the connection to mains ground at PE is removed then Cx and Cy form a capacitive voltage divider across the mains and the metal case is at about 1/2 Vmains. Cx and Cy are typically 1 nF and the current available is below what which is deemed a cardiac hazard by most regulations. The shock which can be received is annoying and the voltage can damage electronic equipment.

Adding a ground lead remedies the problem.

From Wikipedia image here

Providing a ground wire to eqpt metal parts will fix the above problem. This SHOULD be via the mains outlet but a wire to local ground will work much the same. There is some small risk that the wire to random ground will increase the hazard - but unlikely. Wire to power point ground should be safe.

The following answers that I've given to other SEEE questions address the above capacitor related issue:

• Electric shock from laptop due to capacitive coupling?
• Why would a power supply unit with insulating case and galvanic separation need a grounded power cord?
• Plug in charger in laptop before plugging in mains outlet
• USB port and bench power supply ground difference

______________________________

Q&A:

Misunderstaood said

• If someone is getting shocked the voltage is high enough to potentially be lethal. Non-lethal voltages generally do not give shocks with dry skin. 9V will tingle on the tongue. The reason they are not electrocuted is usually due to low skin conductivity. Over 100V is considered lethal. Lethal is more a function of body resistance and current path through the body, e.g through a heart muscle

A: IF the mechanism is the two Y capacitors then by design and regulatory requirement the current is low enough to not cause atrial fibrillation (or any other cardiac issues) in several standard deviations from the mean population - and possibly quite a lot more than that. I'm not saying that it's a good idea or that it shouldn't be fixed.

The impedance of the capacitors is the main factor in current magnitude with a well grounded user. Current is ~~~<= 2.Pi.f.C.Vpeak or around 0.1 mA per nF with 230 VAC mains. Earth leak breakers designed to meet regulatory requirements to protect against cardiac arrest typically trip at 30 mA imbalance (higher than I'd consider wise). Even at 10 mA trip that's about 100 times what will be obtained via the Y caps on 230 VAC.

I'm personally unhappy with the 'higher than previously' limits set for AC & DC ELV in recent standards, despite the technical explanations given as to why they are appropriate.

Answer 2

Most probably it is bad design, not something being faulty in there; however of course quality of assembly matters and someone assembling the device must have sufficient skills and training, otherwise such a thing may happen. But to my understanding, it is out of you control/influence and your knowledge.

I would agree with Christopher Estep in workplace SE that you should not take initiative you are not qualified for and formally charged with. If something will go wrong, you will be the potential scapegoat and there may be a risk to even being forced to jail.

What you should know about at your technical level is ingress-protection. Devices out there have to be certified to be operated in specific conditions (e.g. open air, environments of moisture, under rain, etc). Most probably devices you talk about should have a much higher International Protection (IP) marking than those operated within buildings, and the device should have been certified and must have appropriate IP marking on it.

If a device shocks people in a specific environment, this raises the following questions:

• Was the device certified to operate in these conditions?
• Was the device developed to operate in these conditions?

These are NOT your questions, but your management questions.

The first thing you must do when noticed of this risk to customer health is inform your boss in writing so that s/he will be then in charge of issue resolution unless s/he will order you to fix it (and then you will decide if you are qualified enough to fix it).

Further whistle-blowing is up to you. But you should know by whistle-blowing you can lose your job, and potentially risking your further employment. However, if you do everything per process, you may be considered as a hero.

So:

• Inform your boss, if no proper (from your point of view) reaction, then;
• inform boss of your boss, if no reaction, then;
• go to executive people responsible for sales and legal (legal is important, because if someone will be shocked there will be a lawsuit or even criminal action against company), if no reaction, then;
• you go to authorities.

All these should be in writing, paper or email, and you should get a response in writing when possible.

In the conclusion: Electric/electronic safety is a subject being taught in educational institutions, and a course runs for a quite a long time. I am afraid, while you may get an idea what is wrong with the device, you will be hardly qualified to fix it.

But there's a good side of all this story. If you are so bothered by this case, take these safety classes; who knows, maybe you will become a great expert in it.

P.S.: I will tell you a story somehow related to yours. At the beginning of 2000s I was working for HP, and their field people identified that there was possibility of electric shock from devices called PDUs (power distribution unit) in the racks. HP invested in investigation and fix of this issue, and invested in its services organization to replace all PDUs in the field around the world. Every engineer was given a very sensitive phase detector to identify faulty devices to identify already failed PDU and protect self while working on the system. Thus in general, having this issue uncovered internally and fixed has many positive aspects (1) there will be no upset customers and no lawsuits regarding this issue; (2) there will be no damage to manufacturer's brand and service organisation brand; (3) the service organisation will have additional funding for fixing the issue.

P.P.S.: You will most probably find good technical advice from other answers. Be warned, that if you choose to implement them, you will be ultimately responsible for consequences, not that person who answered. That's why I deliberately refrained from considering your situation from a technical point of view without precise technical information provided.

Answer 3

Yes, it is faulty wiring.

Use a voltmeter between the metal surfaces and a solid ground such as a copper water pipe. You can use the ground on an outlet but if you are getting shocks this may be due to faulty outlet wiring such as the outlet's ground is not really grounded.

Measure both AC and DC.

Cutting ground wires is somewhat horrible. More stupid than horrible.

Cutting ground wires can "fix" the problem because you usually must be connected to ground to get a shock. If not ground, another voltage potential.

Answer 4

First I must say, all the above answers are good if the problem is indeed caused by an internal issue to the machine.

However, there is also the possibility that the issue here is not being caused by the machine, but by you yourself. Is this actually static discharge.

You need to figure out for certain whether the shock is being caused by static discharge or not. The fact that removing the ground from the plate fixed the problem makes me wonder if that in fact is the issue here. Moreover, since he had a ground to disconnect, it sounds to me like there IS a ground system available within the box.

Someone asked you in the comments about the nature of the shock, whether it is crack and a nip or a sustained tingle makes a lot of difference. The former is normally static, while the latter is a more serious continuous current source.

Static can be caused by the movement of the user, and there is not much you can do about it. However, it can also be caused by the machine itself. You did not give us much details of what this machine is, but machines that contain moving parts, especially fans, belts and the like, can generate a LOT of static. Again, proper grounding to a verified EARTH ground and ensuring there are no islands of charge on the casing should fix that.

Ultimately, someone needs to determine which issue is at play here. Put your concerns in writing to management to make it official, and if management is not willing to invest the time to investigate the issue, then I suggest you contact the appropriate regulatory body and have them come out and investigate.