Adding earth and RCD to two pin inverter - Page 1
Forum section: electronics / general

author: motorhomer
author_rank: Contributor
timestamp: August 08, 2021, 04:36:41 pm
content: Hello all, I need some help to clear up a question which was asked about two pin inverters and adding an earthIf I have an inverter 12v to 240v which only supplies a positive and neutral output, how would someone add an earth to the output so they can install a RCD.I was thinking like industry installation the earth will be connected to the neutral at the output end next to the inverter and then an RCD will be fitted live and neutral further down. I cant see any reason this would not work.Would it make any problem grounding this earth to a vehicle chassis with the 12v negative also grounded to the vehicle chassis

author: BrokenYugo
author_rank: Super Contributor
timestamp: August 08, 2021, 05:04:33 pm
content: A RCD/GFCI doesn't need an earth connection to function, they work by checking for a current imbalance between live and neutral, doesn't matter if the leak is to earth or elsewhere. In fact in the USA where we often build them into the socket it's permitted to install one in an old 2 wire installation with no earth connection, works great.

author: Zero999
author_rank: Super Contributor
timestamp: August 08, 2021, 05:21:42 pm
content: Quote from: BrokenYugo on August 08, 2021, 05:04:33 pmThat's not true. An RCD or GFCI does need the mains to be earthed to work. It will work for appliances which aren't earthed, but it definitely requires a connection between the neural of the mains to ground.Quote from: motorhomer on August 08, 2021, 04:36:41 pmThe neutral should be connected to the chassis, in order for the RCD to work. The problem is the inverter will most likely consist of a DC:DC converter, with an H-bridge on the output to convert it to AC. The problem is, the DC:DC converter might not be isolated, in which case, you need an RCD which will work with DC.

author: David Hess
author_rank: Super Contributor
timestamp: August 08, 2021, 05:57:00 pm
content: Quote from: Zero999 on August 08, 2021, 05:21:42 pmIt needs a conductive path between the mains and earth for a failure to occur to be detected, which means that it works fine without it because then there is no possible failure. An imbalance between the hot and neutral currents means that there is a third path, which could be through earth ground or not, and that is what the RCD or GFCI detects.If you had an RCD or GFCI installed after an isolation transformer, then it would detect a simultaneous ground fault and failure of the isolation transformer. Some isolation systems monitor the voltage across the isolation transformer to detect failure so they can disconnect when conditions become hazardous.

author: motorhomer
author_rank: Contributor
timestamp: August 08, 2021, 06:37:06 pm
content: Ok so I understand how the RCD works. With out an earth the only way it will trip is a leak from positive back to UK inverter without going through the RCD. So back to neutral side. With the earth it's more likely to go back this route and trip quickerSo my theory is ok to connect the earth to neutral at the inverter output and then the RCDNot sure about any issues connecting this earth to chassis found some info where it's adviced to do this.

author: motorhomer
author_rank: Contributor
timestamp: August 08, 2021, 06:39:44 pm
content: So agree then nuatral to chassis which will be the earth return. Not sure why it would need to be DC one when the output is AC.

author: Zero999
author_rank: Super Contributor
timestamp: August 08, 2021, 09:24:10 pm
content: Quote from: David Hess on August 08, 2021, 05:57:00 pmYou've missed the point. Without the secondary of the inverter connected to earth, the RCD will not detect a live to earth fault. To be more specific, the neutral needs to be earthed before the RCD, in order for it to work. If it's earthed after the RCD, it will provide no protection.Quote from: motorhomer on August 08, 2021, 06:37:06 pmYou mean phase, or live, not positive, because it's AC.In a car, the chassis is considered to be earth. Some inverters have the neutral connected to earth, some don't. The easiest way to find out is to check the continuity between the neutral and the 0V connection to the inverter. This should obviously be done with the power off.

author: motorhomer
author_rank: Contributor
timestamp: August 09, 2021, 06:16:17 pm
content: Ok so I did get wrong terminology,I agree the RCD will not work without the earth and the way I was going to overcome this was to connect the neutral direct from the inverter to an earth wire giving the three live earth and neutral connections. This will then pass through an RCD and to any appliance so the appliance will have an earth. I do not see any issue with this at first until you get a live conductor short out to the vehicle bodywork then there is a potential lethal voltage on the bodywork which will not be detected until a person touching this body work then touched the earthed appliance.To over come this connecting the earth/neutral connection to the chassis will make it all safe, but then someone pointed out some inverters are not true isolation between input and output so when the neutral switches to high it will be a short to the battery negative.On more expensive inverters there will be an isolation like a transformer I expect. So cheap inverters are either to be kept with no earth to make them safe or do not use them

author: james_s
author_rank: Super Contributor
timestamp: August 09, 2021, 07:32:31 pm
content: This is one case where it probably makes more sense to refer to "ground" rather than "earth". In a rolling vehicle there is no connection to earth unless you're plugged into shore power. The vehicle chassis is ground but it isn't earth ground.

author: motorhomer
author_rank: Contributor
timestamp: August 09, 2021, 07:33:33 pm
content: Ok thanks for that

author: motorhomer
author_rank: Contributor
timestamp: August 09, 2021, 07:45:49 pm
content: So the RVDs look good and a good way to protect on this system. I assume these are not on sale in the UK. Has anyone fitted these

author: Zero999
author_rank: Super Contributor
timestamp: August 09, 2021, 09:44:20 pm
content: Quote from: james_s on August 09, 2021, 07:32:31 pmGround also implies earth to me. They are different words for the same thing. Ground, is more commonly used in the US and Canada and earth, in the UK and Australia. For example, in the UK modern cars are referred to has having a negative, rather than positive earth, as was the case for some vintage models.If you really want to be unambiguous, you should refer to 0V, common, or chassis, rather than earth, or ground.Quote from: motorhomer on August 09, 2021, 06:16:17 pmThe problem is, it's possible the output from the inverter floats, at a DC voltage, with respect to earth/the 0V input terminal, which would result in a short circuit, if you connected the neutral to the chassis.

author: motorhomer
author_rank: Contributor
timestamp: August 09, 2021, 10:17:18 pm
content: QuoteThanks I realise the problem with this if the converter does not have an isolating output. We use large inverters at work but they have isolating transformers on the output stage so the negative can and is connected to a chassis for a earth return.I expect this is the problem with buying cheap inverters. The guy that was asking be the question is now considering buying a different inverter which supplies the correct protection circuits and a earth point