1
\$\begingroup\$

I have an 12V to 230V inverter in my car but get confused about ground it. The inverter is centre-tapped, means live-earth voltage is +115V, neutral-earth voltage is -115V, this gives an L-N voltage of 230V. Earth is connected to battery negative terminal, which I confirmed with multimeter. Live/neutral is not connected to anything on DC side. The inverter is powered by a secondary battery, which connects to starter battery via voltage sensitive relay. The negative posts of two batteries are always connected regardless of relay state, means the inverter Earth is connected to car chassis. However, this raises me some concern about safety:

  • Since both live and neutral have potential relative to car chassis, I will get electric shock if touching any wire. Would it be better to use an isolated DC-DC charger instead of connecting secondary directly to starter battery?

  • I have an RCD on the inverter output, which should theoretically trip if live/neutral touches appliance earth, or me touching live/neutral. But if I remove the connection between starter battery and secondary battery (make inverter a floating system), I will not get shocked when touching live/neutral, and RCD still works, right?

This problem confuses me for a long time because grounding the inverter doesn’t seem to provide any advantage. Instead, increases the risk of me getting electric shock.

Update:

I connected the inverter output to an RCD block, then connected a 10W LED flood light to L-E pins of the RCD block. The LED light turned on and RCD does NOT trip. I swapped the connection on LED flood light to N-E, and the light still illuminated, nothing happened to RCD. This proves that a centre-tap inverter will NOT trip an RCD if live/neutral is leaking to earth. Considering the inverter earth is connected to vehicle chassis, I cut the earth connection inside my inverter, makes it an IT system.

There is a guy on YouTube also tested RCD on an inverter, and the result is the same as mine. see link here

https://www.youtube.com/watch?v=GTqnkT_tO30

More expensive inverters may come with neutral-earth link. In this case, I believe it is safer to have an RCD on its output, then connect inverter earth to car chassis. IT system has its own disadvantages; in case of live touches car chassis, it still develops fault current through capacitive coupling. Having an RCD that quickly disconnect power source is better than letting it leak.

See Victron's book for more details about inverter earthing: https://www.victronenergy.com/upload/documents/Wiring-Unlimited-EN.pdf

\$\endgroup\$
2
  • \$\begingroup\$ So what inverter make/model is it and what does the installation manual say how it should be installed? \$\endgroup\$
    – Justme
    May 5 at 13:37
  • \$\begingroup\$ I think the question is basically the same as grounding in house wiring. Grounding it does increase the chance of you getting an electric shock by touching live or neutral.... but it also decreases that chance. All metal parts you can touch should be connected to ground, so if live or neutral touches one of those parts, it blows the fuse. \$\endgroup\$
    – user253751
    May 5 at 16:02

1 Answer 1

0
\$\begingroup\$

There's danger of shock to personnel coming in contact with either of the lines and chassis.

Hence the RCD to safeguard against that.

enter image description here

\$\endgroup\$
11
  • \$\begingroup\$ What's a two-pole RCD and why won't a standard 230V RCD work with standard 230V inverter? \$\endgroup\$
    – Justme
    May 5 at 18:40
  • \$\begingroup\$ Is there a way to test RCD? The button works fine but that means nothing. I tried RCD tester from local electronic shop but it refuses to test, because N-E voltage is greater than 50V. \$\endgroup\$
    – Jason
    May 6 at 0:36
  • \$\begingroup\$ Hi Jason, The RCD button test ascertains the functionality of the tripping mechanism. The RCD tester, on the other hand, checks the neutral-to-earth resistance / voltage and aborts the test should that not be low enough to get an accurate test result. \$\endgroup\$
    – vu2nan
    May 6 at 6:11
  • \$\begingroup\$ I've heard that RCD won't trip properly on a centre-tapped inverter. That is probably because although L-E has voltage, but the waveform may contain DC component and/or not sinusoidal, which renders RCD useless. L-N voltage is isolated from DC side and is sinusoidal, but N-E and L-E involes switching MOSFETs, thus waveform is unpredictable. \$\endgroup\$
    – Jason
    May 9 at 14:39
  • \$\begingroup\$ @Justme - Hi, In a standard, single-phase RCD, a single line and neutral are monitored whereas in a two pole RCD with a pig tail, two lines and neutral are monitored. The inverter, with its 2 lines and a neutral, is equivalent to the American split-phase system. Having a 2-pole RCD, with its pigtail connected to chassis, safeguards against shock to personnel between either of the lines & chassis. If the inverter had had only one line and a neutral, a single pole MCB would have sufficed. \$\endgroup\$
    – vu2nan
    May 10 at 5:27

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.