Isolation transformer is flipping gfci on new install

Had a similar head-scratcher last year when I was setting up the emergency backup system in my motorhome. Fitted a Victron isolation transformer to separate the shore power earth from my onboard system — sensible precaution, or so I thought — and immediately had the GFCI on the hookup post tripping every time I connected.

Took me a good few evenings to work it out. The issue was that my transformer secondary was floating (as it should be), but I'd accidentally bonded neutral to earth after the transformer, which confused the GFCI into thinking there was a fault current. Once I removed that bond, everything settled down.

A few things worth checking if you're in a similar situation:

• Is your secondary neutral bonded to earth anywhere downstream? It shouldn't be with an isolation transformer in the circuit
• Check for any stray leakage paths — even a dodgy appliance plugged into the secondary can cause enough imbalance to trip a sensitive GFCI
• Victron's own documentation on isolation transformers is actually quite thorough on this — worth a proper read rather than a skim

I imagine this kind of thing comes up on boat installs quite a bit too, where you've got multiple transformer banks and shore power splits getting complicated fast.

Anyone else navigated this with a multi-transformer setup? Curious whether splitting across separate breaker banks introduces its own complications — feels like the kind of thing that could get messy quickly if the earthing isn't sorted from the start.

@PaddyDavies classic case of the transformer's internal capacitance giving the GFCI something to cry about — mine did the same in the static until I realised the neutral-earth bond on the secondary side was fighting the one upstream like two stubborn blokes over the last sausage roll.

@VanKen already covered the capacitance weeping, so I'll spare you the repeat — what nobody's mentioned yet is that some GFCI units are just allergic to isolation transformers full stop, particularly the cheaper American-spec ones that have no business being near a UK install anyway.

Had the exact same nightmare in my static caravan last winter. Swapped out the offending GFCI for a proper RCD (you know, the thing we actually use here in Britain) and it sorted itself immediately.

If you're dead set on keeping the GFCI, check the Victron manual — there's a minimum load threshold before the transformer behaves itself and stops leaking enough ghost current to upset sensitive protection devices.

Basically: wrong protection device for the setup, mate.

@NotAnElectrician80 don't leave us hanging mate — "some" what? Your post got cut off!

But while we wait for the cliffhanger resolution... quick question for the thread: does it matter which GFCI you're using? I've got a similar setup planned for my shepherd's hut and I've seen people saying some cheaper units trip at 4mA while others tolerate up to 30mA before they get hysterical about it.

Presumably if the transformer's capacitance is bleeding enough to tickle a sensitive one, the fix might be as simple as matching the right GFCI threshold rather than rewiring half your install?

Or am I wildly oversimplifying this and about to electrocute a sheep? Genuine question.

@Robbo ha, the suspense is killing me too! While we wait for @NotAnElectrician80 to finish their thought — one thing worth checking that hasn't come up yet is the GFCI's trip threshold. Standard household units trip at around 5mA, but marine/RV-rated ones are typically set at 30mA to account for exactly this kind of leakage from isolation transformers and long cable runs. If you're using a domestic GFCI on what's essentially a marine-style install, that could easily be your culprit right there. Worth swapping for a proper marine-spec unit before pulling your hair out over the transformer itself.

@HalfAJob93 worth adding to your point — I went through almost this exact saga with my Fogstar-fed 48V system last spring. The culprit turned out to be the transformer's interwinding capacitance coupling a leakage path to earth through the GFCI before the system had even settled.

What fixed it for me was adding a bleed resistor across the secondary — about 100kΩ between L2 and earth — which gave that capacitive leakage current somewhere to go without tripping the GFCI. Cleared it immediately.

Also worth checking: is your GFCI a 30mA unit? Some cheaper ones are genuinely too sensitive for transformer installs. Victron's own documentation quietly acknowledges this. Swapping to a 10mA-rated unit specifically rated for transformer loads made a noticeable difference on a mate's panel van build.