Effect of ZS and RCD trip Times...!

[DurhamSparky]

questions!!!!

chatting to some body today and well they were going on about ZS can have effect on RCD trip times!! is this true? never heard about it before..!

any body clear this one up??

 

[DurhamSparky]

any body!! ???

 

[rocker]

Hmmmm I reckon if you have a ludicrously low PFC this is going to have a detrimental effect on an RCD trip time, but that's off the top of my head and I don't have the science to back it up.

I reckon your mate is probably talking so much balls.

 

[DurhamSparky]

thought so!!! lol..!

reason i ask is that i took a Type C RCBO 20A out of a DB as it was nuisance tripping! (the circuit is on a radial with ZS of 1.88 at last point and its in an office powering PC's..! IR Readings = >200 all round and its all run in metal basket / trunking underfloor feeding the floor boxes..!

every thing been fine for last 10 years! all of sudden its giving Nusance tripping! i Ramp Tested the RCD and it Blew at 15mA rather than 30mA....!! chap was blabbering saying well you ZS is higher than stipulated thats why its half the trip amount! the readings from DB schedule and my ZS is actually 0.2 Ohms more than when it was back in 1997....! any way i personally think that my RCD is Broken and have quoted for a new one (or new model one any way as they no longer for sale) just decided to ask!

 

[DurhamSparky]

Hmmmm I reckon if you have a ludicrously low PFC this is going to have a detrimental effect on an RCD trip time, but that's off the top of my head and I don't have the science to back it up.

I reckon your mate is probably talking so much balls.

lol, Ex Collegie from Mr Electric! lol

 

[rocker]

Nah that's 100% a dodgy RCD. Is this fella still with your previous employer, cos that sounds like their sort of hairbrained logic.

I reckon rip off merchants start to believe their own rubbish after a while.

LEGAL DISCLAIMER: I am not saying Mr Electric are rip off merchants. I do think they're rubbish, though.

 

[DurhamSparky]

nah works for Mittie now! he had a disagreement with the gaffa! lol...!

LEGAL DISCLAIMER: I am not saying Mr Electric are rip off merchants. I do think they're rubbish, though. <<<<<<<<<<<<<----------------- hahahahahaha!!

 

[Mark_Burgess]

I am sure the RCBO has been regularly tested as well.

I wonder if the amount of added PC equipment over the years has a combined earth leakage that is near or over the 30mA mark or over 15mA and the RCBO is faulty as you have found.

And i supposed it has not been wired with high integrity earthing in mind!

But hey, YOU are the electrician who just checked and tested it so YOU are obviously wrong.

 

[DurhamSparky]

lol...! they have 5 pc on two banks of desks! 5 x 3.5mA = 17.5mA just enough to dump a 15mA RCBO!! would have been fine if 30mA was available! lol...!

 

[rocker]

LEGAL DISCLAIMER: I am not saying Mr Electric are rip off merchants. I do think they're rubbish, though. <<<<<<<<<<<<<----------------- hahahahahaha!!

Well, you know what happened last time!

 

[nick611]

why rcd on computer circuit.asking for trouble.trained or supervised persons,commercial enviroment,no rcd required.ps zs theory rubbish!!

 

[nick611]

make sure its not overload.not only earth fault that trips on rcbo.no heaters pluged in is there?thats a favourite in an office .rcd faults very rare

 

[Doug]

chatting to some body today and well they were going on about ZS can have effect on RCD trip times!! is this true? never heard about it before..!

every thing been fine for last 10 years! all of sudden its giving Nusance tripping! i Ramp Tested the RCD and it Blew at 15mA rather than 30mA....!!

On a course I was told that regularly operating / testing an RCD will gradually lower the tripping current. At the training centre, the RCDs on the test boards were tested so many times a week that they had to be replaced every few months. Perhaps your RCBO is just getting old and needs replacing.

I can't see how the tripping current would be directly related to Zs at the RCD. A very high Zs could mean that the tester fails to generate sufficient earth fault current to trip the RCD, giving an error reading. This should not be a problem with Zs of only 1.88 Ohms. After all, on a TT system with Ze of 100 Ohms you would still expect an RCD to pass a ramp test.

chap was blabbering ......
any way i personally think that my RCD is Broken and have quoted for a new one

I think you answered your own question there.

 

[DurhamSparky]

cheers fellas! just needed to nail that stupid theory from his gob shut tbh!! lol..!

quote for new RCBO and Busbar link plus out Hours fitting! £140!! woop woop! stupid Merlin Gerin Stuff!! lol..!!
seen a direct replacement on ebay for £20 quid lol...! hmmmmmm. maybe not!

dont think it is overload as its one of them RCBO with the 2 Flick Toggles where Overload drops but RCD dont but RCD pulls MCB Toggle with it!

Merlin Gerin V40H 1N 20 - 20a 30mA Type C RCBO on eBay (end time 03-Nov-10 07:25:09 GMT) <<<---- identical! one no longer made now though!

 

[paul_sussex]

it makes perfect sence that zs values can have a detrimental effect on rcds, howether in this case it is minor.
r1 and r2 values have an effect on ressistance of current therefore a resistance on fault current whether on live conductors or earth.

 

[malcolmsanford]

why rcd on computer circuit.asking for trouble.trained or supervised persons,commercial enviroment,no rcd required.ps zs theory rubbish!!

Not sure you are quite on the mark there to be honest. How is an room full of office workers classed as skilled or instructed persons.

You might be better using the second part of the 411.3.3 as labelled or identified sockets intended for a particular equipment ie computers.

Though nothing wrong with RCD IT equipment. The old 16th recommended as a guide that you should allow for a 25% calculation for equipment using high protective conductor currents so a 30mA RCD would allow for approx 8mA of leakage. Rule of thumb today is perhaps 8 items of equipment for each RCD.