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If part of your PCB carries high voltages at certain nodes, is there a standard that requires some sort of silkscreen label or sticker to warn technicians when installing/uninstalling boards into products?

Lets consider high voltage to be any voltage that can cause moderate harm to a person.

I am from USA.

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    \$\begingroup\$ NFPA 70E standard focuses on electrical safety in the workplace. While it primarily deals with safe work practices and procedures, it may also include recommendations for labeling of electrical equipment, including PCBs. \$\endgroup\$
    – liaifat85
    Apr 22 at 16:21
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    \$\begingroup\$ Consider that the person you cause moderate harm to from lack of a warning might be able to find out who you are. (And you live in a country where random nutters are allowed firearms.) \$\endgroup\$ Apr 22 at 17:36
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    \$\begingroup\$ @AndrewMorton My primary concern is production line workers and service techs. \$\endgroup\$
    – BobaJFET
    Apr 22 at 17:42
  • \$\begingroup\$ @AndrewMorton I’m just getting the joke now… At least in my defense I could say I warned them. \$\endgroup\$
    – BobaJFET
    Apr 22 at 17:57
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    \$\begingroup\$ Is anything going to be live when technicians install/uninstall this? \$\endgroup\$
    – Graham
    Apr 23 at 20:25

8 Answers 8

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I don't think you really need a standard to help you decide whether you should put a warning label on a PCB; I think you can use common sense and put one on even if the voltage might not be generally regarded as something that is unsafe. You could even state what that voltage is to cover all the bases. If in doubt, put on a warning label. It costs you virtually nothing to do it.

enter image description here

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    \$\begingroup\$ What sort of label would be applicable? Aside from the “high voltage” symbol you typically see near substations for >1000V, is there some other universal symbol for dangerous voltages? \$\endgroup\$
    – BobaJFET
    Apr 22 at 17:46
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    \$\begingroup\$ @BobaJFET on a recent PCB (for in house use) I drew several skull and crossbones alongside some words that explained there was a danger of death from electrocution. I didn't find an adequate symbol that enhanced the message I was putting across but, I expect there may have been if I'd looked hard-enough. The PCB handled several kV so, there was no doubt in my mind that a big (and repeated) warning was necessary. \$\endgroup\$
    – Andy aka
    Apr 22 at 17:51
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    \$\begingroup\$ There is a standard symbol for high voltage consisting of a lightning bolt inside of a triangle. The Mooretronics font set has symbols useful for electronics. The high voltage symbol in the Mooretronics set is H. \$\endgroup\$
    – qrk
    Apr 22 at 22:04
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    \$\begingroup\$ @qrk Isn't that reserved for true "high voltage" sources greater than 1000 Volts? I try to avoid using regulated symbols like that unless it's applicable. \$\endgroup\$
    – BobaJFET
    Apr 23 at 12:12
  • \$\begingroup\$ @BobaJFET I see this symbol stamped on aluminum heat sinks for mains connected power supplies we use. I use this symbol on areas of our PCBs that are above 48 V, mainly so I, or our test staff, don't do something stupid in that area when testing boards. If you've ever gotten across 350 VDC, you will bless the warning symbol. \$\endgroup\$
    – qrk
    Apr 23 at 14:12
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IEC61010: Safety requirements for electrical equipment for measurement, control, and laboratory use

PCBs with hazardous voltages should be protected with an enclosure, and should not be reachable during normal use.

There is a general requirement that if the enclosure contains hazardous voltages, the enclosure itself must contain a warning to disconnect power. For capacitors there is a requirement of discharge to safe voltages within 10 seconds of removal of power.

There is an exception "If it is not feasible for operating reasons to prevent the following parts being both ACCESSIBLE and HAZARDOUS LIVE" .. "parts intended to be replaced by an OPERATOR (for example, batteries) and which may be HAZARDOUS LIVE during the replacement or other OPERATOR action, but only if they are ACCESSIBLE only by means of a TOOL and have a warning marking".

The warning marking is the triangle with lightning symbol, combined with text such as "Caution, possibility of electric shock" or a more specific text.

That said, even though the standard assumes that users would obey the warning to disconnect power before opening the device, someone may end up troubleshooting it live. So go ahead and add extra warnings on the PCB, even when they are not required by the standard.

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    \$\begingroup\$ Do they define what "harzardous" voltages are? \$\endgroup\$
    – BobaJFET
    Apr 23 at 12:13
  • \$\begingroup\$ @BobaJFET Yes, in quite a bit of detail; but in short 70 VDC @ 2 mA or 33 VAC @ 0.5 mA. There are more specific limits related to charged capacitors and RF energy, and separate limits in case of single fault conditions. \$\endgroup\$
    – jpa
    Apr 23 at 18:09
  • \$\begingroup\$ @BobaJFET Since you specify current, must I measure the resistance of the gun owning service technician to determine if the current will be exceeded with the applied voltage? \$\endgroup\$ Apr 26 at 1:14
  • \$\begingroup\$ @richard1941 If I understand your question correctly, no. You are allowed to have up to 70 VDC source at any current. Alternatively you are allowed to have higher voltage source as long as the source is current-limited to less than 2 mA. As soon as you have more than 70 VDC available at more than 2 mA load current, it is classified hazardous. \$\endgroup\$
    – jpa
    Apr 26 at 6:19
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Generally there should be a protective element preventing general public, machine operators and service personnel from getting into contact with a live part.

That means that the PCB should be protected by a hatch, fence and a gate or what ever achieves that. The warning label should be well visible on the protective element, for example on a hatch that would protect from "high" voltage. Even if the hatch was transparent, it would be difficult to rationalize the warning label being on the PCB.

Once you remove the protective element with the label (open a hatch), you are exposed to live circuits and there is no requirement for further warnings.

If there is an specific care point, where attention should be drawn, for example if there is only one point exposed or if one point might be energized after shutdown. Using too many warning labels might confuse the operator, who might fail to identify another live part.

In the US the warnings should follow the ANSI color scheme and style, where as in EU a lightning symbol in a yellow triangle is usually used. Exceptions to the colors, text and symbols on the labels might be allowed, but compliance testing house would request rationalization if you only used the PCB and silk screen color for the warning.

The exact requirement depends on the end use of the product. Maybe there is an application (Defense/aerospace/medical) where you would have to access live circuits or "high" voltage batteries that cannot contain other labeling. If in those cases the only mean of mitigating the risk of injury As Far As Reasonably Practicable, would be to use silk screen, then it would become a requirement.

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For military applications, the requirements for labeling are given in MIL-HDBK-454C section 4.8: "danger, warning, caution, signs, labels, tags, and markings should be used to warn of specific hazards such as voltage, current, thermal, or physical. The signs, labels, tags, and markings should be as permanent as the normal life expectancy of the equipment on which they are affixed. Guards, barriers, access doors, covers, or plates should be marked to indicate the hazard which may be present upon removal of such devices. When possible, marking should be located such that it is not removed when the barrier or access door is removed. Additionally, hazards internal to a unit should be marked adjacent to hazards if they are significantly different from those of surrounding items. Such a case would be a high voltage terminal in a group of low voltage devices."

Labeling is required for more than 70 V: MIL-HDBK-454 Table 1-II

Note that MIL-STD-1472H section 5.7.8 states that "All electrical systems of 30 volts or more are potential shock hazards. Research indicates that most shock deaths result from contacts with electrical systems ranging from 70 to 500 volts."

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Consider enlightened self-interest

As other answers have said, standards focus on labelling the outside of the case. Anyone authorized to assemble/disassemble/repair your equipment must be assumed to be suitably trained.

That said, you and your fellow engineers are going to have to develop, commission and test this equipment before it gets as far as the production line. Can you, at a glance, tell which parts of the board are safe to touch or attach an oscilloscope probe to, and which are not? If you can't, I strongly suggest you label it so that you can. The person you're saving from "moderate harm" could well be yourself!

Our equipment uses moderately high voltage to drive piezo actuators. There's a dashed silk-screen line on the board, with the bitey side indicated with lightning bolts. So far no-one has got themselves shocked (although we've had a close call with one manager).

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Perhaps you might search because there is a lot of government information on the subject. First, define High Voltage.

2.1. High Voltage: Any voltage exceeding 1000 V rms or 1000 V dc with current capability exceeding 2 mA ac or 3 mA dc, or for an impulse voltage generator having a stored energy in excess of 10 mJ. These current and energy levels are slightly below the startle response threshold (IEEE Trans. Power App. Sys., vol PAS-97, no. 6, 2243, November, 1978)

2.2. Moderate Voltage: Any voltage exceeding 120 V rms (nominal power line voltage) or 120 V dc, but not exceeding 1000 V (rms or dc), with a current capability exceeding 2 mA ac or 3 mA dc.

Source: EEEL Safety Rules for Moderate and High Voltages

Unless it is 1000 volts or more, it is not high voltage. Voltage is not what kills you; it is the current. 10 mA will kill you, 1 mA makes you mad and cuss.

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    \$\begingroup\$ 1 mA in the right place can kill you too, though. \$\endgroup\$
    – Hearth
    Apr 22 at 15:59
  • \$\begingroup\$ Yeah, if you stick multimeter probes beneath your skin like the fabled story goes. The anecdote reads some navy guy tried after taking an electronics class and then died. \$\endgroup\$
    – Voltage Spike
    Apr 22 at 17:34
  • \$\begingroup\$ Adding on to what hearth points out, take the resistance of human flesh into account (1000 - 10000Ohm) and you’ll see that it can take a lot less than 1000V to kill. \$\endgroup\$
    – BobaJFET
    Apr 22 at 17:48
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Do you see such markings on PCBs in equivalent products from reputable manufacturers?

I've seen plenty of teardowns of products like power bricks, PC power supplies and similar. Any of which, in the EU, might be handling 230v which is potentially fatal.

Power bricks are often glued shut during manufacture. They usually don't have a 'high voltage' warning either on the PCB or on the enclosure. However, they are generally glued shut during the manufacture process, and there's no reasonable expectation anyone will touch the PCB at all, let alone touch it with the unit powered on.

ATX power supply sticker

ATX power supplies will commonly have a sticker saying, among other details, that there are hazardous voltages and no user-serviceable parts inside. It isn't generally on the PCB, it's on the outer enclosure. But again, there's no expectation users will come into contact with the PCB or mess around with it while the power is connected.

On the other hand, if your product is something like a solar inverter, where it's expected that an installer will unscrew certain panels in the field to hook up cables - you'll often see additional warnings. Because it's expected, in the normal course of installation, that installers will come into contact with the mains voltage screw terminals.

Of course, silkscreen doesn't cost anything - so you could chuck some warnings on the PCB if you've got the free space, even if they're not required by law.

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This is not an engineering problem!

I would put the warning label on the PCB even if no dangerous voltages are present. Then, no matter what idiocy takes place beyond your view, you cannot be accused of failing to warn. Warning label should also say, "Only qualified technicians should handle this circuit board."

And remember: a certain nation is also full of voracious lawyers who are far more evil and can cause far more pain and suffering than mere gun owners.

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