RCD trips at 7 am some mornings

[Gilgamesh59]

I have an RCD protected mains supply that trips the RCD at 7:00 am exactly on some, but not all, mornings. I moved into this house a couple of mths ago, and initially it was tripping almost always on Mondays, but last week it tripped on Mon, Tue, and Fri.
Neighbours advise that previous occupiers had the same problem, ruling out my appliances. The house is 2 storey detached and 7 years old, so I doubt if any significant modifications to the electrics have been done since.
The RCD is a 30 ma type and self test button trips it ok. Its not connected to the lighting, which has its own set of CBs but no RCD. Power is 240v from underground supply.
I thought it may have been a fault with the evaporative cooler, but I unplugged that in the ceiling this weekend and on Monday the RCD didn't trip - I thought I'd nailed it! - but on Tuesday morn it tripped so back to the drawing board.
The house electrics include: Evap cooler and gas ducted heater (Brivis) both controlled from a wall mounted thermostat/controller. Electric oven, one electric element on the cooker, range hood, and automatic garden sprinkler system.
The sprinkler system controller is unplugged and not used.
The only timing system in the house that apparently preserves its time when power is interrupted appears to be the heating/cooling controller (maybe it has a battery backup?).
This is driving me nuts! I am trying to isolate the issue to one circuit now, by switching off CBs each morn just before 7am and watching for a trip, but due to the unpredictability of the trip, a failure to trip doesn't prove that I've isolated the fault.
Any help greatly appreciated, and FYI, I am located in Melbourne Australia.

 

[holmslaw]

G'day.

I would suspect this is something to do with your heating/cooling system and the clock is set to turn it on at 7am. And the intermittent tripping depends on the status of the controls.

Simple to prove check the time settings and change from 7am to 8am.
If this changes the tripping time you need a spark to trace the fault.

Any electric clocks in the house? unplug them.

 

[kkynaston]

I think you need to be carefull in oz with electrics as the average bruce or sheila is only allowed to change a light bulb.

If you don't get an electrician for everything else you will end up in court.
So whilst tracing the fault, don't go opening up and plugs, sockets, junctions, fuseboards.

 

[JohnD]

Turning of the MCBs may not help, if they are single-pole ones (as is usual over here) since you may have have a neutral-earth fault.

Unplugging will isolate appliances.

the heating/cooling is quite likely the source, and as you surmise, something controlled by a timer. Earth leakage most commonly comes from watery appliances like kettles, steam irons, boilers, water heaters, electric showers, also from heating elements like ovens, also from things which get wet, such as outdoor lighting, pond pumps, outdoor sockets, and equipment in outbuildings. Personal computers also generate earth leakage, worse if you have several.

A photo of your consumer unit/distribution board would be interesting to see how it is laid out and what circuits are protected by how many RCDs. What is the sensitivity of your RCD (ours are usually 30mA or 100mA) and which circuits do you lose when it trips?

However, it is also possible for a trip to be caused by an event outside your property, such as a neighbour turning on/off something or a nearby industrial unit or shop starting up/closing down equipment

 

[Gilgamesh59]

The RCDs here are 30 ma. I think the CBs are double pole, but will check that. There is only one RCD, it protects all the power CBs but not the lighting CBs.
Yesterday I changed the time setting on the wall controller 15 minutes ahead, to see if that changes the tripping time. This morning it didn't trip, so I'll have to give it a few days...
That's the frustrating thing! Knowing only the time, but not knowing which days it will trip and when it won't!