Onboard SLA Battery Charger / AC Mains earthing interaction - Page 1
Forum section: electronics / general

author: NE666
author_rank: Frequent Contributor
timestamp: October 08, 2024, 12:15:31 pm
content: Not an Electric Vehicle question but an 'electricity in vehicles' one, if I may. I'll also readily concede that this is really a question for the technical support team of the proposed supplier of the equipment concerned but I'm getting no response from that avenue.I'd like to install a permanently mounted 12v SLA battery charger/maintainer in my car and combine that with an external weatherproof AC power input port to supply 'shore power' to the charger when parked on my driveway. I'm also considering making the AC supply available elsewhere within in the vehicle, to allow the operation of a small plug-in PTC fan heater for preconditioning during the winter months.My question concerns the earthing of the AC supply within the car and the effect it would have on the charger. The short version is; in my proposed installation scheme the negative DC pole of the charger would be connected to its neutral AC line input via the car's bodyshell. As far as I am currently aware, the charger as-is a) is double insulated and thus has no earth connection and b) the DC output is galvanically isolated from its AC input (this path shows off-scale high impedance when measured). So this isn't the 'normal' configuration.For context, the reasons this situation would come about are;1) Where I live in the UK, the mains supply uses the TN-C-S PME earthing scheme (meaning the neutral line and earth conductors are bonded and hence equipotenial)2) For safety, I would connect the earth terminal of the AC input port to the car's metal chassis3) The car uses the standard 12v chassis negative schemeTaking these together, we can see that the charger's -ve DC output lead would be connected to AC earth and neutral, via the car's bodywork. Whereas it wouldn't, if simply connected to the battery and powered by something like an extension lead.Does this create any problems? Or is it typical? For further clarification, this is not an electric vehicle but a regular ICE one, so no other on-board equipment connected at any time to the mains supply.

author: DenzilPenberthy
author_rank: Frequent Contributor
timestamp: October 08, 2024, 12:52:48 pm
content: Hi, short answer is yes, this creates problems.It is not safe to export a TNCS earth outside of your house's equipotential zone.In normal operation there wil be some potential difference between TNCS earth and true terra-firma earth, due to the current flowing in the combined neutral and earth (i.e the TNC part of the circuit). Under fault conditions of a broken neutral this can rise up to full mains voltage. This very nearly killed me when I was a kid. A broken neutral in our supply and I was outside in bare feet playing with the hose. I got a huge electric shock when I touched the garden tap.Connecting your car body to the TNCS earth exposes you to this risk. It is better to leave the installation in the car floating and not connected to the car body. As the battery charger is double insulated, you are not exposed to the risk of touching true earth and the TNCS earth at the same time. Make sure your heater is also double insulated.

author: Ian.M
author_rank: Super Contributor
timestamp: October 08, 2024, 01:03:18 pm
content: +1Yes its a problem but *NOT* because the charger output /battery negative is grounded.Quote from: NE666 on October 08, 2024, 12:15:31 pm*THIS* is the problem - its extending the TN-C-S PME equipotential zone of the house to the car which during fault conditions (including supply side or neighbouring property faults) may differ considerably from the local ground surface potential. Add a wet driveway and metal door handles and its electrocution waiting to happen.In theory at least a properly installed TT earthing system for the outside socket solves the problem, but the problem is 'properly installing' the TT system as the earthing rod(s) cant be near any buried pipes or cables that connect to the TN-C/TN-C-S earth. Also if you *ever* plug in away from home, pray the socket you are using is a proper TT supply.John Ward has some UK centric videos that may be worth watching:

author: NE666
author_rank: Frequent Contributor
timestamp: October 08, 2024, 01:24:09 pm
content: Thank you both. I really should have known better, having actually watched John's presentations on the subject of exported earths some time ago. I was fixating on the charger's circuitry.So, no earth connection to the chassis and let it float, whilst using only double insulated equipment within.Given that it is my intention to only every connect to the supply at my home, is there any worthwhile safety improvement to be gained by making the AC supply to the vehicle available through a suitably sized isolation transformer?

author: DenzilPenberthy
author_rank: Frequent Contributor
timestamp: October 08, 2024, 01:58:52 pm
content: I wouldn't say it was worth the hassle or expense of isolation transformer.If you really wanted to you could get a dedicated socket installed that's on a TT earth but see above for the trap of installing a TT earth rod too near other things.If your system is floating and not connected to any exposed conductive parts, like the car body or appliance cases, then you're good.But in any case, do make sure that the supply to the car is RCD protected.

author: Ian.M
author_rank: Super Contributor
timestamp: October 08, 2024, 03:07:11 pm
content: A RCD wont save you if the incoming PE conductor isn't at local earth ground potential and you use any non-double insulated equipment, e.g. a laptop charger with a three wire mains lead. If there are any mains sockets 'aboard' you either need an isolating transformer so you can run a local isolated TNC system, grounded to chassis, or you need an earth fault current detector monitoring the incoming PE that will disconnect *all* three conductors if there's a fault. If using 'shore power', driving a ground rod whenever you can would be a good idea, and if that's not possible, maybe use an aluminum grounding sheet you roll a wheel over and hook to the chassis so anyone entering or exiting doesn't get the first 'belt' that trips the earth fault detector.

author: NE666
author_rank: Frequent Contributor
timestamp: October 08, 2024, 03:56:46 pm
content: Quote from: Ian.M on October 08, 2024, 03:07:11 pmThanks, I like that idea. Not least of all since I already have a sizeable mains isolation transformer in my 'parts bin'. There's no extraneous conductive metal in the driveway either; overhead power, no gas supply, water on MDPE ("blue") pipe, neighbouring properties the same.

author: DenzilPenberthy
author_rank: Frequent Contributor
timestamp: October 08, 2024, 04:24:10 pm
content: The RCD isn't anything to do with the earth voltage issue. It is to protect against shock from cable damage etc.

author: Ian.M
author_rank: Super Contributor
timestamp: October 08, 2024, 04:26:49 pm
content: Yep. You *NEED* RCD protection on the fixed TT outlet, and if you implement an on-board isolated TNC system, that needs RCD protection downstream of the Neutral to chassis bond.

author: f4eru
author_rank: Super Contributor
timestamp: October 10, 2024, 11:55:42 am
content: Quote from: Ian.M on October 08, 2024, 03:07:11 pmIt can be done, but not a good idea.bulky, lossy, anda lotcan go wrong from the safety point of view.Why not just simply take a class II charger instead ? it's much safer, the secondary is meant to be touched.If ever you want extra safety, you can simply add an RCD before, if your home isn't already equipped.

author: Ian.M
author_rank: Super Contributor
timestamp: October 10, 2024, 12:47:18 pm
content: Yes, I certainly wouldn't want to lug around the weight of an isolating transformer unnecessarily, as unless its a very large vehicle the extra weight will significantly impact fuel economy, so a permanently installed class II charger is a preferable option, but if you have 'on board' mains sockets there is a risk that class I equipment will be used, which opens up the whole TN-C-S equipotential zone export hazard can of worms again. An isolating transformer is the old-skool way to mitigate that hazard when a TT supply outlet cant be provided (or if you don't trust that the outlet *IS* a properly earthed TT supply). There are earth fault current detector systems that can disconnect all the supply conductors including PE, but they are fairly rare and usually either for big three phase systems or built into some complicated EV charging point. Single phase ones in a simple DIN rail form factor an ordinary 'sparks' can safely install seem to be about as rare as rocking horse s--t!

author: NE666
author_rank: Frequent Contributor
timestamp: October 10, 2024, 07:23:03 pm
content: Quote from: f4eru on October 10, 2024, 11:55:42 amWould you care to elaborate?

author: NE666
author_rank: Frequent Contributor
timestamp: October 10, 2024, 07:36:19 pm
content: Quote from: Ian.M on October 10, 2024, 12:47:18 pmWhat I am proposing is to install (within a suitable enclosure) the transformer within my garage, wired into a fused spur from the existing RCBO protected supply to the wall sockets. Its output would be externalised via a weatherproof external socket (RCD protected?) using the TT earth. I'd use a suitable extension lead from there to the 'shore power' socket installed in the car.Quote from: Ian.M on October 10, 2024, 12:47:18 pmIs this such a device?https://www.cef.co.uk/catalogue/products/5050064-single-pole-auto-reset-pen-fault-protection-with-100a-mcb

author: Ian.M
author_rank: Super Contributor
timestamp: October 10, 2024, 10:08:03 pm
content: There's not a lot of point in having an external isolating transformer when you've got a TT supply with known good earthing you've personally seen tested. It only makes sense for connecting to dubious power hookups when out and about.Matt:e are one of the few manufacturers doing this stuff. The particular device you linked is BS:7671 2018, Amendment 1 2020 Regulation 722.411.4.1 (iv) compliant, which means it senses a PEN fault by looking at the Line to Neutral voltage and tripping if that goes out of spec. It also serves as MCB for over current protection. The datasheet is a little light on the details, but it looks like it doesn't include RCD protection (i.e. detecting Line vs Neutral current imbalance), and may well not monitor actual PE conductor current. Its likely to suffer from nuisance tripping if the shore power voltage is low e.g. due to excessive lead length. You'll need to talk to their technical department to see if it offers useful protection for an in-vehicle installation.