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So I'm creating a backpack system to carry around and shine a 100 watt HID lamp (using the arc tube for UVC mineral collecting.) This system currently includes a 12v lithium battery, a power inverter to 120vac, and a 120v ballast setup (including a 300vac cap) that drives the HID. The actual voltage output to the HID is 20-30vac.

With all these different voltages, I'm a bit confused about ground. Should I have some sort of common ground in this system, or should every part be isolated?

Again, all this will be sitting in a backpack while the light will be in hand. So also figuring out whether I should build a box containing these things out of aluminum or something non-conductive instead like wood.

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  • \$\begingroup\$ Be careful with UVC. It can damage your skin. \$\endgroup\$ – Elliot Alderson Apr 7 '20 at 23:21
  • \$\begingroup\$ CONSIDER a system with metal case grounded to 12V negative, with 12V positive well isolated from user contact making 12V user contact very low possibility. Then float the AC supply. Make it very hard to contact either AC leg. || In outdoor situations if you get good contact between user and ground and between one DC battery pole and ground and if user makes body contact with the other DC pole you CAN get user bad muscle lockup. I know of it happening to a friend standing in water (flounder fishing, 12V LED light) - much less likely with your application. ... \$\endgroup\$ – Russell McMahon Apr 8 '20 at 2:35
  • \$\begingroup\$ ... Locking all 12V+ inside a full metal 12V- shell makes it "impossible" - keep it that way !!!. - Having AC floating allows one lead contact without harm. The same would be true of the battery BUT AC is much less liable to cause can't let go shock. | DO NOT common DC and AC sections for same reasons. \$\endgroup\$ – Russell McMahon Apr 8 '20 at 2:36
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CONSIDER a system with

  • Metal case grounded to 12V negative, with 12V positive well isolated from user contact making 12V user contact very low possibility.

  • Then float the AC supply. Make it very hard to contact either AC leg.

In outdoor situations if you get good contact between user and ground and between one DC battery pole and ground and if user makes body contact with the other DC pole you CAN get strong user-muscle-lockup.
I know of it happening to a friend standing in water (flounder fishing, 12V LED light) - much less likely with your application.

Locking all 12V+ inside a full metal 12V- "full metal jacket" makes contact "impossible" [tm] - keep it that way !!!.

Having AC floating allows one lead contact without harm.
The same would be true of the battery BUT AC is much less liable to cause can't let go shock.

DO NOT common DC and AC sections for same reasons.

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  • \$\begingroup\$ So it sounds to me like you're saying the most dangerous part of this system is for one of the AC legs to make contact with the DC circuit. Is this true? And by floating AC supply, you mean having it mounted on an insulator, not making contact with anything else? \$\endgroup\$ – Jared Cravens Apr 8 '20 at 3:41
  • \$\begingroup\$ Srt of but no. || Neither is overly dangerous but prudent avoidance is prudent. || Joining them electrically has no real advantage and some disadvantages. Maybe place a 10k resistor between DC -ve "ground" and one AC leg to bleed off any electrostatic that may happen. 10k will not be a shock hazard of any sort. | AC is safer at a given voltage and even at somewhat higher voltages. || The 120 VAC is nasty safety wise and should be kept totally away from human contact or fault risk. . \$\endgroup\$ – Russell McMahon Apr 8 '20 at 4:08
  • \$\begingroup\$ || Floating AC supply means that IF THE HID circuit allows do not connect the transformer output to anything else but the bulb. The HID striking system may not allow this. \$\endgroup\$ – Russell McMahon Apr 8 '20 at 4:08
  • \$\begingroup\$ So far the output of the transformer only has an igniter between it and the HID. Thanks for all your help \$\endgroup\$ – Jared Cravens Apr 8 '20 at 11:48
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It really depends on if there are isolation transformers in the system, because a fault could go through you instead of back to the source. AC doesn't really care where it goes, it will go to a lower potential, if that happens to be you, then you could be electrocuted by dangerous (above ~60V) voltages.

If you want to be safe would probably be best to have some kind of chassis ground around each component and all the grounds tied together. In consumer products it is common to have a chassis ground for faulting currents and a way to break the current (fuse or breaker) in the event of a fault.

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  • \$\begingroup\$ So the battery negative, one leg of the 120v, and a leg from the 300v at capacitor will all be making contact with each other through a common ground? That's not a short? Also, from what I understand, a ground circuit in AC ultimately sends the fault to the earth. So can you really have a "ground" inside a backpack? \$\endgroup\$ – Jared Cravens Apr 7 '20 at 22:19
  • \$\begingroup\$ No, just provide conductor around all components (does not need to be connected to any components) so there is no chance of a fault current outside of the conductive shell. \$\endgroup\$ – Voltage Spike Apr 7 '20 at 22:35
  • \$\begingroup\$ So if I'm stuffing all these components inside a wooden box, line that box with metal? What about fuses? Will that also help protect me in the event of a failure? \$\endgroup\$ – Jared Cravens Apr 8 '20 at 0:01
  • \$\begingroup\$ You should put fuses in between the devices so that if a fault occurs, the fuse will blow. \$\endgroup\$ – Voltage Spike Apr 8 '20 at 2:52
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The question of earthing would not arise, interconnection would be simpler and the backpack lighter, should you put together a modular system comprising of a battery and an automotive 12V DC 100W HID ballast having cables with moulded connectors.

Standard automotive interconnection cables, with in-line fuses, would take care of input and output short-circuit protection. There would be no question of isolation, with the battery having a plastic housing and only the ballast a die-cast aluminium enclosure.

The system could be housed in a light-weight aluminium enclosure with the ballast mounted inside for improved cooling. Should cooling not be an issue, a lightweight, flame-resistant and rugged polycarbonate enclosure could be considered.

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