View Full Version : RCD Fault

24-11-2007, 07:48 PM
Hi there got a mitsubishi SRK 50 Hyper Inverter ZGX, Installed it 3 months ago working fine but for some reason now keeps triping the RCD all the wiring seams fine and dry, unit pulling correct current so decided to rewire it with out the RCD on seprated circut with 16A fuse box it runs fine on hot and cold any ideas way it is doing it??? got me lost could it be the rcd is just too sensitive

The Viking
24-11-2007, 08:21 PM
RCDs should only be used to protect stuff that can electrocute humans, never for fixed mechanical installations.

An inverter driven compressor is likely to trig a RCD under normal working conditions.

Unless of course you meant MCB....

24-11-2007, 08:24 PM
is the mcb if it is a mcb motor rated? type C/D

24-11-2007, 09:28 PM
cheers guess thats why its working fine now, love the a/c thing guess thats why love playing on my home just wished i could learn more about it

25-11-2007, 07:41 PM
Hi this is a really common problem with inverters see below
In domestic and light commercial buildings it is common to have a device called an RCD residual current device, commonly called a trip switch which is used to protect the electrical circuits and the users of them.

The RCD works by measuring the current in the Live wire and the Neutral wire and comparing them, in a healthy circuit the currents will be equal. In an unhealthy circuit some current leaks to Earth.

If there is a difference of more than 30mA the switch trips and everything goes
Off. This is not a fault with the aircon unit so donít send out new parts.

The problem with RCDs is that computers and Inverters all have some earth leakage so in some cases if the total earth leakage exceeds 30mA the RCDS can be nuisance tripped by the inverters.

If this happens there are two courses of action available to you

1- replace the RCD with a less sensitive model up to 100mA RCDs are ok in domestic installations
2- Wiring the Aircon unit so it doesnít pull current through the RCDs.

hope this helps

regards Hendrag lg tech manager

25-11-2007, 09:04 PM
is the mcb if it is a mcb motor rated? type C/D
D rated breakers should only be used on circuits with transformers and not motors.

The tripping curves are too high for motors and do not offer the correct protection.

06-01-2008, 10:37 AM
The manufacturer will be able to tell you the total leakage current for the unit. Hitachi I've just put in (not currently working!!!) has 40mA max leakage.

Also because of the high leakage a seperate 4mm2 earth cable should be connected to the outside unit's chassis and taken directly back to the instalations MET.

This is to ensure leakage does not result in a PV rise on the outdoor units case, which could cause a shock.


06-01-2008, 01:35 PM
Air cons should not be on a RCD in the first place. RCD's are to protect people that may be touching something that comes live due to a short.
Indoor splits are plastic, outdoor units are protected OK by the fuses, and neither need to be held or touched in order to operate them.
Air cons are a PITA when hooked up to an RCD. I've been to an old fuji that trips a RCD, nothing is wrong with it, just the CF motor is about 1Mohm with a megger to earth. That's borderline for a motor but causes no dramas. But because it's on a RCD the customer has to buy a new motor or get a sparky out to rewire.
Solution is simple for a new install, when putting them in wire the c/b to the mains before the RCD.
edit: also I've seen a lot of PCBs and inverters that do leak a bit of current and the manufacturer is aware it's a part of the design, so it's normal as far as they're concerned

14-01-2008, 03:57 PM
Semi blanket statement here but all electronics pcb's which have an emi filter section have a current path to earth as there always caps that bleed to earth in this part of the board. Do some Googling on X-Y capacitors and emi suppression and you will find a lot of stuff. This page is good reading and the circuit diagrams make it easier to understand.

Sometimes it's just the aircon gets blamed for being the straw that breaks the camels back when there are numerous devices in the circuit as all things with emi filters have these caps. I remember commissioning a VRF system a while back that had 16 Indoors on the one power circuit that was fed from a standard domestic 16A 30mA MCB/RCD. Most times when the indoors were powered up the RCD would trip but a bit of creative work with the power connectors on the pcb's (leaving half unplugged initially, then plugging them back in 1 by 1 after flicking the breaker on) would allow us to get the indoors powered up and the job commissioned before the site electrician could get back to change the RCD to one with a slower trip time to allow for the inrush as all indoor pcb caps charged up and also with a higher mA tripping capacity.

That was after they had already tried to get us to commission the system when it was powered by a gen set (A big sucker of a machine), that thing just spat the dummy the moment either the outdoors or indoors were energized as its controls were uber sensitive.