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comptonspark

Hi have a problem which I hope that somebody can shed some light on.

New install on existing TT supply, with two DBs fed from an SWA sub main with 100mA S type RCD and 100A switchfuse at the origin (no other loads on supply).

DB 1 is for a new install and all circuits are protected with 30mA RCDs. DB2 is a new board for a number of existing circuits again all 30mA RCD protected except for two further sub mains which are just on MCBs with 30mA RCDs at the far end. Both boards and sub mains etc test out ok and 30mA RCDs all trip in the required times.

The problem occurs when DB1 has all circuits on and some load (mainly IT equipment and some lights (its a small office) and DB2 is commected with the main switch on and the two RCDs on but all MCBs are off. After a while the 100mA S type RCD trips. It seems ok with only one RCD on but not both. Can also get some MCBs on before the S type trips.

I've tried checking connections, checked wiring, retested IR of circuits, but to no avail, I still can't seem to find out what is causing the trip. Even clamped the tails at the S type and only get 10mA to 15mA leakage and the max hold function dosen't record any larger values.

So anybody got any suggestions? I was thinking of getting another S type and trying this otherwise I'm scratching my head on this one.

thanks
 
ramp test gave 55mA and trip time of 267 ms. tested leakage at load side of S type, just outside of the enclosure. I'm not too sure of the ramp test, just used the standard Megger1552 ramp function (does this work with S types?)
 
Have you tried removing connected loads and trying again?
I had a similar fault with a tumble drier, used to trip after 2mims exactly. Everytime. Lol.
If it clears it reconnect loads one at a time
 
When you say the max earth leakage was 10 to 15 mA.

Was that with all loads connected and was it when the RCD tripped?

55mA on the ramp test passes the RCD so does the 267ms as its under 300ms if it is a EN BS RCD. (Is that the highest of both sides)

The megger 1552 does allow you to ramp a S type its just it has a built in 200ms delay.

Are you selecting S type on the megger before testing ?

If you max leakage recorded is 10 15 mA I would look at either a loose connection on either side of the 100mA RCD (maybe insde a henly block or switched fuse unit)) or possibly a faulted unit.

I would of liked to see a higher Ramp value than the one you recorded!
 
yes the leakage was with the loads connected,that is with the same loads connected as when it tripped.

I'll have to look at the megger manual and try the ramp test again,I just used the 'normal' ramp test. I think I'm back to a more detailed look for loose connections or parts in the switchfuse RCD or Henley as you suggest.
 
Have you done an IR test on your new install? I would of thought you have!

Im thinking of a possible N/E leakage as you said its ok with 1 RCD but not 2 - have you tried to turn the 2nd RCD on first.
Have you got any load connected to this RCD
Do a basic PAT test on all appliances (set to 250v) and see if you get good values.

It sounds like there is a problem with the new install and not the S-Type, but saying that 55mA is a little low for a 100mA RCD.
 
yes the leakage was with the loads connected,that is with the same loads connected as when it tripped.

I'll have to look at the megger manual and try the ramp test again,I just used the 'normal' ramp test. I think I'm back to a more detailed look for loose connections or parts in the switchfuse RCD or Henley as you suggest.

Thanks

With the 1552 the Ramp test is performed the same as normal and will display the trip current make sure you select the 100mA current mode.

The time delayed mode is as follows. Select the 100 mA mode and press and hold the small blue button on the side untill it beeps and the flashing warning exclamation mark appears in the window,then carry out the test on both sides of the sign wave with AC displayed.

You will only be able to test on both sides on 1 x not on the 1/2 x current!

Hope this helps

Let us know how you get on.

Regards

Tony
 
Sounds like a nightmare Friday afternoon scenario, if you find the cause please post, thanks.
 
went back to site on Monday and re IR tested all circuits and remade connections, all tested ok and with no obvious loose cables or connections etc. Could manage to get both DBs up and running with loads on , but the 100mA S type still tripped when a certain string of 5 HF Twin fluorescent fittings was turned on (another string of 3 was already on and not causing problems) so disconnected first one of the five and then a second and behold the S type held in. I had already ramp tested (trips at 65mA) and trip tested (trips after 327ms) this device and as you can see it tested ok.

So went back Tuesday and changed the front end RCD to a new (MK) 100mA S type and reconnected all fittings etc and everything now OK (well so far so good).

Conclusion: there were certainly some small faults with the replacement board and the existing circuits, but re testing and remaking the connections appeared to clear these, thus allowing for the 'real' fault to appear. (despite torqueing the neutral bar screws thery still didn't seem to really hold the connections properly.)

I guess that the original S type just didn't like some of the (possible) interference/ HF transients or what ever, generated by the HF fittings (or for that matter the attached Solar PV system) and these just added up and caused the tripping. During the whole process none of the 30mA RCDs (covering every circuit) ever tripped so I now doubt that any leakage or earth faults were the cause of the problems.

It would have been nice to be able to prove this conclusion but I don't know if there is actually any affordable test equipment available that I could have used?. Changing the RCD for a different model seems to have cured the problem. So I'll write this one down to experience.

thanks for the helpful advice from all, it certainly helped to focus on some possible causes rather than clutching at straws!
-Michael
 
Cottonspark, you are only confirming what i had already predicted, ..a faulty RCD device!! On the large projects that i'm mainly involved with, you get to see first hand just how many unreliable and/or faulty RCD devices there are out there, (And we use premium manufactures too)!!

Makes you shudder, that so many electricians out there will openly rely on RCD protection rather than sorting out a circuits or an installations Zs values (ADS)....
 
Aha - you only just mentioned the Solar PV system :)

Depending upon the Inverter used, they have very special RCD requirements - especially on a TT system, because some of them can and do leak a residual current and some can also leak DC Voltage, so that could well have been influencing the results that you have been getting. - there are pages and pages of discussions about RCD's TT and Solar PV over on the Photovoltaic / Solar Panels & Green Energy Forum ...
 
I had a similar problem with a new 100mA TD RCD tripping before a new 30mA when a HF fluorescent light was turned on. Replacing RCD fixed fault. I sent faulty RCD back to manufacturer who would not believe me that there were no circuit faults, also sent full test results of circuits, ramp tests, and RCD tests. They sent me two 100mA TD RCDs back. I assume it was noise from the HF fluorescent which failed as soon as I left. The two RCDs did not make up for the time I had wasted
 
Cottonspark, you are only confirming what i had already predicted, ..a faulty RCD device!! On the large projects that i'm mainly involved with, you get to see first hand just how many unreliable and/or faulty RCD devices there are out there, (And we use premium manufactures too)!!

Makes you shudder, that so many electricians out there will openly rely on RCD protection rather than sorting out a circuits or an installations Zs values (ADS)....


Agreed you should alway try to fix the cause of an abnormally high Zs value on TN systems, but on a TT system providing it's a stable reading <200ohms then it's within spec - so we just have to keep fingers crossed and hope the rcd works if it is needed:teeth_smile:
 
but on a TT system providing it's a stable reading <200ohms then it's within spec - so we just have to keep fingers crossed and hope the rcd works if it is needed

Rather you than me on that score!!... I'd never accept a 200 ohm Ra on any TT system!!
 
With having all RCD protection on the boards the only thing you needed to check was the feed cable really ,as they would of tripped before the 100ma one. Unless I'm reading how it was all connected together wrong lol. Glad it's sorted anyhow.
 

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