20kVA generator trips consumer unit RCD - Page 1
Forum section: electronics / general

author: Chris Wilson
author_rank: Super Contributor
timestamp: August 19, 2023, 12:21:27 pm
content: Modern 20kVA single phase diesel generator in a domestic environment. Gennie is AVR controlled and makes 235V at 50Hz with a decent sine wave. Loads connected directly to it run fine.The electrician tidied / rewired the outside electricity company supply box and added a second box with a break before make two pole transfer switch in it. He wired it with suitable cable to the workshop and fitted a 125 Amp socket in the workshop to plug the generator into. What I find is the generator makes power and although the house lights work, the consumer panel RCD trips. I think only the sockets are on the RCD as all the house LIGHTING circuits work OK on the generator.The generator has neutral and earth bonded n the alternator box and it has the chassis grounded to a six foot earth spike near the machine. The outside electricity board box states the supply is a multiple earth system. What would cause a presumed earth loop that would trip the consumer panel RCD please? The electrician is away abroad on holiday. .... Thanks for any ideas, I'm tempted to isolate the generator earth spike and see if it fixes it (just as a test).

author: themadhippy
author_rank: Super Contributor
timestamp: August 19, 2023, 01:10:21 pm
content: Theres a couple of alarm bells ringing about the installationQuoteIn modern money thats known as a TNC-S supply and earth spikes are not permitted to be installed on the consumers side,the chassis bond should go to your main earthing terminalQuoteI do hope that's just a terminology slip up and there's a plug (inlet ) fitted in the garage and you electrician isn't expecting you to use a widow maker to connect your generator

author: Stray Electron
author_rank: Super Contributor
timestamp: August 19, 2023, 01:10:31 pm
content: Let me be sure that I have this right, There is nothing connected to the output side of the RCD? Does your RCD unit have GFI built in as well? What happens if you connect a light resistive load (such as a 100W bulb) to the RCD output before turning on the power?I've never used an RCD unit but my understanding is that (in the US) they monitor both L1 current and neutral current and trip when the two currents are not equal. Groundingshouldn'thave anything to do with their operation. In Britain you use L1 and L2 for power so I'm assuming that the RCDs there monitors those two currents instead of L1 - N.Just a thought, do you have anything on the output side of the RCD that might have MOVs in it? I'm wondering if the inductive voltage surge caused when you apply power might be causing a MOV to turn on momentarily and that is causing a current imbalance. You might try disconnecting the wiring on the output side of the RCD and see if it still trips. If it doesn't then that could mean that the inductance of the wiring itself is causing a voltage spike.

author: Chris Wilson
author_rank: Super Contributor
timestamp: August 19, 2023, 01:52:31 pm
content: My terminology was bad. The "socket" in the workshop is a wall plug (male pins) and the lead from the generator is a flying socket! (Female receptacles).I tried disconnecting the generator earth rod, no difference. With it disconnected the bond between neutral and generator chassis inside the alternator connection box would then go via the power cable from the generator to the supply company's ground in their incoming mains box.I'll try with everything in the consumer unit turned off and see if I can narrow things down, thanks again

author: themadhippy
author_rank: Super Contributor
timestamp: August 19, 2023, 02:02:03 pm
content: QuoteAn RCD is a GFIQuotegrounding has everything to do with there operation,without a decent ground connection there's no path for the fault current to travelQuotenearly all domestic supply's in the uk are single phase,or Live and Neutral

author: Jeroen3
author_rank: Super Contributor
timestamp: August 19, 2023, 02:28:38 pm
content: If you make a drawing of your installation the error will become obvious.

author: Stray Electron
author_rank: Super Contributor
timestamp: August 19, 2023, 02:40:47 pm
content: Take a look here at some of the issues with RCDshttps://www.electriciansforums.net/threads/rcd-trip-when-power-cut-occurs.206917/

author: Chris Wilson
author_rank: Super Contributor
timestamp: August 19, 2023, 03:06:04 pm
content: OK, nailed it I think to the consumer unit in the garage. House consumer unit has a 32 Amp MCB that I was TOLD was for the garage, and is marked as "Garage". With that and *ALL* MCB's off on th house CU as soon as i turned the main house consumer unit isolation switch on the main RCD tripped. If I turn the master switch on the garage consumer unit OFF all is OK. If the garage CU is turned on, even with all the MCB's OFF, the house CU RCD trips and it's a 100mA one.I just need to get my head around how the housebuilder wired this stuff up, and why, with all MCB's OFF on both the house and garage CU's but the main CU switches ON, it trips....On normal mains these all work as expected.Anyway the gennie runs EVERYTHING in the house and workshop, including a rotary three phase converter and all the workshop stuff without even getting to half its load capacity, which was the main purpose of the installation testOne MCB in the garage consumer unit didn't like the inrush to an air bottle compressor and has had repeated resets over the years, maybe MCB is knackered but I can't think why it would cause a Live / Neutral imbalance which is what should trip an RCD.Things may be clearer tomorrow <LOL>

author: themadhippy
author_rank: Super Contributor
timestamp: August 19, 2023, 04:07:32 pm
content: Quotein that case i'd start by looking for a neutral - earth fault.

author: geggi1
author_rank: Frequent Contributor
timestamp: August 19, 2023, 05:18:00 pm
content: Quote from: Chris Wilson on August 19, 2023, 12:21:27 pmYour generator is set up for a TCN (4 wire system) system but the grid is designed as TNC-S (5 wire system) the simplest and safest is to modify the generator to TNC-S by splitting the neutral and ground in the generator. You will have to change the socket on the generator from a 4 pin to a 5 pin.

author: Chris Wilson
author_rank: Super Contributor
timestamp: August 19, 2023, 09:58:30 pm
content: Quote from: themadhippy on August 19, 2023, 04:07:32 pmDoes that apply to a here in the UK UK system, where there's Live, Neutral and ground via a three pin connector? I suspect this is more US and wherever orientated

author: Monkeh
author_rank: Super Contributor
timestamp: August 19, 2023, 10:09:03 pm
content: Quote from: Chris Wilson on August 19, 2023, 09:58:30 pmIt applies anywhere you have a separate neutral and earth.

author: IanB
author_rank: Super Contributor
timestamp: August 20, 2023, 02:14:10 am
content: Quote from: Stray Electron on August 19, 2023, 01:10:31 pmYou should phrase that as a question rather than a statement. In Britain, we use L - N for power. A standard three pin plug/socket has L, N and E, where N and E are bonded together somewhere upstream. An RCD (aka GFI) checks for imbalance between L and N.

author: Chris Wilson
author_rank: Super Contributor
timestamp: August 20, 2023, 11:59:28 am
content: It's a fluorescent light circuit in a workshop fed off the garage consumer unit. I used a Megger to test every separated neutral off the busbar until I found one with 400 or so Ohms to ground. Of course it's the lights in my workshop with a car lift in it, so they are hard to get to "up in the Gods", but at least I now only have eight fluorescent fittings to check. Running all the property on the gennie power as I typeI believe the reason it didn't trip on mains is the service supply struggles to get over 220V most of the time, the AVR on the gennie is at 245V, so it generates more leakage current I suppose.Thanks for the help.