Home office - frequent tripping of the RCD

[spenny_b]

Hi Guys,

Just signed-up, so hello to everyone! first post...be gentle (pls)! Apologies in advance for the length of the post.

I have a home office at the bottom of the garden, built this time last year, the power was installed by a local electrical company, but I'm getting constant tripping of the RCD's.

The installation is as follows:

- 30A supply (dedicated circuit) from house consumer unit was already feeding the garage.
- A new 'board was installed into the garage, supplied by the above 30A feed, with a 63A/30mA RCD and a single B20 MCB (Proteus metalclad surface mount unit)
- This MCB feeds the home office (about 20m away) via amoured cable.

- A second consumer unit (same as the Proteus unit above) is installed in the office, with a 63A/30mA RCD and two MCB's; one marked B20 for the ring main cct, the other marked B6 feeding a 2A lighting cct.

- An earth spike is installed directly outside the office, behind the fusebox.

The problem I'm getting is that the office RCD will often trip. I can trigger this by simply plugging in a "kettle" lead into the back of my home PC or monitor (note, not necessarily switching it on, just applying power to the device), sometimes switching on a laser printer will cause it to trip.

I've recently finished building a new PC, using an 850W PSU (Enermax), that also does exactly the same. This morning I was lucky and didnt trip the supply by plugging it in, but as soon as I tried to power it up, "bang", it tripped. Sometimes just the office RCD, sometimes also the garage RCD.

I have two 1kW panel heaters installed into the office, neither of which have tripped the power when coming on via the timer, or cutting in on the thermostat.

The final "googly" is that I'll quite often come in first thing in the morning to find it's tripped overnight. Heaters arent programmed to come on, nothing is powered up, no lights....the only things "active" are a mini hi-fi on standby and a scanner also on standby.

To reset the supply is a helluva game, and definately the point that makes all this completely irritating and now intolerable!....there seems to be no routine that works every time, its a case of walking back and forth to the garage, resetting that one, coming back down, resetting the office RCD, etc, etc..back and forth like a fiddlers elbow, sometimes taking 10-15mins to get it going again, as just resetting the MCB's will always re-trigger the RCD(s).

I've just been checking every socket, both 13A and 2A lighting; all connections are fine, and the way in which they're wired up looks logical to me as well. The connections in each consumer unit also look good, as does the connection to the earth spike outside.

Please help, I'm going mad and not getting any proper work done!

(BTW..the company that originally installed the electrics will not be invited back to look at it, for a number of reasons that I shouldnt go into here)

Do I need a different category MCB? I presume mine are Type B, which I believe have a tolerance for domestic inrush currents? Can RCD's be faulty insomuch as being too sensitive? I know RCD can only be tripped by a short-cct or drawing too much current, but the new PC (above) also has a new PSU. As a test, I took it indoors, and everything behaved perfectly.

I have 13A plug-in Power & Energy Monitor (from Maplin) and have made sure I'm not drawing too much current from each outlet and also in total (although sure of this before, as sometimes it goes during the night with virtually nothing on).

Any help gratefully received!

Cheers

Spencer

 

[JohnD]

(1) it is a poor design, with several RCDs in series to protect the same installation.

(2) you seem to have some kind of earth fault. Try unplugging EVERYTHING and see if it stops. If so, plug things in one at a time and see what does it. If not, disconnect the load circuits (both phase and neytral wires) if you feel competent to do it, from the office/shed CU and see if that stops it. that will tell you if it is a final circuit or not.

(3) An RCD trip is nothing to do with power loading or the rating of the MCBs. They only trip from short circuit or overload; and RCD only trips from earth Leakage.

It is most often "watery" appliances, like kettles, immersion heaters, sometimes cookers. I don't suppose you have many of them in your shed.

Look for signs of water ingress anywhere, or damaged cables (including rodents gnawing them).

There is a chance that the underground cable has been damaged and is getting damp this iwll be very hard to find.

 

[Spark123]

OK, firstly you need to distinguish between an RCD and an MCB. An RCD is a device which looks at the ballance between live and neutral currents and operates if they differ by around 30mA, just like the plug in lawn mower powerbreaker. An MCB is basically a replacement device for a fuse.
Which is tripping? Why do you need to go forwards and backwards to the house? If you unplug everything will it reset? Office equipment can have a high earth leakage current which can cause RCDs to trip, but with only a few items this shouldn't really be a problem.

 

[JohnD]

...RCD can only be tripped by a short-cct or drawing too much current,...

You've got that wrong.

 

[Pensdown]

A few questions;

Do you know what type of earthing arrangements you have in your house; if not could you post a picture of your house CU and intake head?

How far away from the house is the office?

Has the problem been with you from day one?

Has the house earth been taken to the office?

As John has said, it's been very badly designed and the earthing arrangements between the house, office and earth rod may be part of the problem.

 

[spenny_b]

Thanks for the replies so far chaps; I'm hoping to get a few hours sorting this out over the w/e (in between changing daughters nappies!).

I've also spoken to a mates son who's an electrician in Herts; his dad owes me a favour, and his son has offered to come down next w/e if I havent resolved it by then.

John, mates son also commented on the second RCD in the office.

Answering the questions that I know off the top of my head...

The cable run between the garage and office is armoured, although not yet underground, so not diff to check for damage.

In terms of equipment down in the office, theres a fair amount, although its never all active at any one time:

Laptop
2 x Home PCs (and associated peripherals)
2 x Monitors
Satellite receiver
TV
PS2
Hi-Fi
2 x Printers (laser & inkjet)
Shredder
2 x 1kW oil/convection combo heaters

4 x floor lamps (all fed by 2A lighting cct)

No kettles/immersion heaters/cookers....yet...SWMBO is thinking about getting me a microwave and a portaloo outisde, so I dont have to bother coming up to the house...ever...

Distance from garage to office approx 16m. Main CU is in the kitchen, however, diagonally opposite corner of the house (if thats relevant)

Yes, prob has been there since day-1 unfortunately.

I'll check out the earth configuration tomorrow and post findings.

Again, thanks for help so far.

Spen

 

[bernardgreen]

The problem I'm getting is that the office RCD will often trip. I can trigger this by simply plugging in a "kettle" lead into the back of my home PC or monitor (note, not necessarily switching it on, just applying power to the device), sometimes switching on a laser printer will cause it to trip.

The answer lies in the trip happening when the plug is inserted to a known good piece of computer equipment.

Many equipment power supplies have their input filter before the switch so as soon as it is plugged in and before it is switched on there will be the "leakage" to earth due to the capacitors in the filter.

So there is a limit to the number of such power supplies that can be connected to a supply with an RCD ( or RCCB ) My experience is that is is about 10. ( filter leakage can be around 2 to 3 mA and some 30mA RCCBs will trip at 20 mA

Also the in rush current on these power supplies can be very high as some rectify the mains into a capacitor to feed a DC to DC convertor. This capacitor appears as a near dead short until it charges up. Fast acting MCB (fuses) will trip on this.

Use a slow acting MCD for the ring to the office equipment to avoid in-rush tripping. Divide the equipment into clusters ( up to five or six units per cluster ) and have a local RCCB for each cluster. Put those on a ring (or radials) that are NOT on an RCCB in the consumer unit. Put a separate ring on the RCCB for kettles, heaters etc. Feed the workshop from a non RCCB feed from the house or fit a 100mA for the circuit if need to protect cable to the workshop.

That way your computors should stay powered. But also take into account the applicable wiring regulations.

Also think about an emergency light, the type that come on when power fails.

Bernard
Sharnbrook
UK

 

[miajanjua]

The problem I'm getting is that the office RCD will often trip. I can trigger this by simply plugging in a "kettle" lead into the back of my home PC or monitor (note, not necessarily switching it on, just applying power to the device), sometimes switching on a laser printer will cause it to trip.

The answer lies in the trip happening when the plug is inserted to a known good piece of computer equipment.

Many equipment power supplies have their input filter before the switch so as soon as it is plugged in and before it is switched on there will be the "leakage" to earth due to the capacitors in the filter.

So there is a limit to the number of such power supplies that can be connected to a supply with an RCD ( or RCCB ) My experience is that is is about 10. ( filter leakage can be around 2 to 3 mA and some 30mA RCCBs will trip at 20 mA

Also the in rush current on these power supplies can be very high as some rectify the mains into a capacitor to feed a DC to DC convertor. This capacitor appears as a near dead short until it charges up. Fast acting MCB (fuses) will trip on this.

Use a slow acting MCD for the ring to the office equipment to avoid in-rush tripping. Divide the equipment into clusters ( up to five or six units per cluster ) and have a local RCCB for each cluster. Put those on a ring (or radials) that are NOT on an RCCB in the consumer unit. Put a separate ring on the RCCB for kettles, heaters etc. Feed the workshop from a non RCCB feed from the house or fit a 100mA for the circuit if need to protect cable to the workshop.

That way your computors should stay powered. But also take into account the applicable wiring regulations.

Also think about an emergency light, the type that come on when power fails.

Bernard
Sharnbrook
UK

Exactly what I was thinking.
You have got to the core of the problem Bernard!
Best technically Correct reply of the lot.

Well done Mate.