Eratic tripping of RCD

[Biggles..]

RCD is seperate from the board and acting as a main switch but haven't checked out that resistance have IR the tails from RCD to Wylex and they are full scale with and without cbs.
Only one neutral bar in board so no issues there.

Just back from there and only way to avoid it keep triiping is to tem fit a mainswitch instead of RCD until i can get back on monday for further invest and to try and discover where th o/c neutral is on the downstairs ring.

Until hen im just plucking for ideas and everyone elses wealth of knowledge.

 

[securespark]

Eventually traced it to an N/E fault on a ring, but how the other circuit got involved is a mystery.

placing a heavy load on the non-rcd side will increase the potential difference between neutral and earth on the RCD side and thus drive more current down your N-E fault.

Interesting theory, plug! But what do you call heavy? 500W?

 

[securespark]

only way to avoid it keep triiping is to tem fit a mainswitch instead of RCD until i can get back on monday for further invest and to try and discover where th o/c neutral is on the downstairs ring.

I hope it's not a TT....

 

[mst]

good question????

is it?

 

[Biggles..]

As mentioned in first post PME(ze 0.12), but good point.

 

[securespark]

Sorry, you did say!!

 

[Biggles..]

its cool, been a long day and this job is starting to get annoying in that its getting harder to find the time to get it right.Diary is going crazy already, thought id done well to get up to date before new yar, first week back already 1 day behind. Here comes another sat and sun

 

[Adam_151]

Interesting theory, plug! But what do you call heavy? 500W?

Could well be, if you assume a ze of 0.5ohms (and assume that neutral is the same CSA as phase in the street main) then it'll take a potential difference of 15 or so millivolts between earth and neutral to drive enough current through the interconnection to trip the RCD

The potential difference between the two would of course depend on the resistances and currents flowing in the section of wire in each conductor from where they joined (and thus no potential exists at that point) to the fault (so its not the just the load in your installation that matters, unless it happens to be TN-C-S) , it is also important to consider that at the fault point, the earth as well as the neutral may be at a different potential than the neutal-> earth bond [wherever that happens to be], because it too carries some current in the real word, and in the case of TT this may be quite substantial because you have got the suppliers electrode in the circuit which may be upto 21 ohms (guess what happens if someone supplied by the same rural transformer decides to test a new earth electrode by the ELFI tester method and you have a neutral -> earth fault...!)

 

[plugwash]

Interesting theory, plug! But what do you call heavy? 500W?

that depends

in an installation with a neutral to earth fault there are two paths that current can take to get from your non-rcd neutral back to the earth/neutral branch point (your cutout on a TN-C-S, further back on a TN-S), the designed path (through main switch neutral, neutral tail etc) and the fault path (through the RCD neutral, the neutral of the faulty cuircuit, the neutral-earth fault, the earth of the faulty cuircuit, the main earth etc)

if the two paths are similar in resistance (say a lead sheathed TN-S supply with a dead short N-E fault close to the CU) then the current that needs to be drawn from the non-rcd side to cause a trip would be arround twice the RCDs trip point (its actual trip point which i belive is supposed to be between half its rated trip point and its rated trip point), if the fault path was significantly lower resistance than the designed path (say because the neutral tail was also bad) then the current needed from the non-rcd side to cause the trip could be almost the same as the rcds trip point.

at the other end of the scale a fault where the fault path was significantly higher resistance than the designed path then the current needed to cause the trip could be much much higher.

 

[securespark]

It was TN-C-S on that job.