'No trip' function. High readings??

[Silk6911]

Hi guys just joined.

Using Megger 1553 multi tester. Great tester but when using the no trip function to get my loop imp reading through an RCBO the readings are really high...15-20 ohms?

Any ideas. I believe this is a common complaint on a lot of testers?

Cheers

 

[Carter]

Any ideas. I believe this is a common complaint on a lot of testers?

Do you mean all testers or just Megger's?

 

[PAUL M]

i have metrell multi tester and i have the same problem,i was told because it uses less current to do the test in order not to trip the rcd it is slightly less acurate,its like if you mearured a football pitch with a 1 inch ruler or a ten foot ruler the smallest method of measurement will be less accurate thats the analogy i was given by manufacturer.

 

[Shakey]

Hi guys just joined.

Using Megger 1553 multi tester. Great tester but when using the no trip function to get my loop imp reading through an RCBO the readings are really high...15-20 ohms?

Any ideas. I believe this is a common complaint on a lot of testers?

Cheers

well this is common but it doesnt matter

as long as the reading is 1667 ohms or less, it complies

 

[sparkyork]

can somone explain something: in the regs theres the table outlining max earthloop readings etc for varying mcb's, fuses etc.....so if you gotta circuit who's Zs should be below say 1.7 ohms then whats this 1667 ohms all about??

really confused? does this mean that loop readings on circuits are pretty much redundant?

 

[Shakey]

can somone explain something: in the regs theres the table outlining max earthloop readings etc for varying mcb's, fuses etc.....so if you gotta circuit who's Zs should be below say 1.7 ohms then whats this 1667 ohms all about??

really confused? does this mean that loop readings on circuits are pretty much redundant?

A subject close to my heart Rich, the answer is pretty much yes for RCD protected circuits.

1667 ohms is the result of dividing 50V by the trip curent of 30mA. (50V being the maximum safe touch voltage)

so what we are saying is, if the Zs is 1667 ohms, when the fault current reaches 30mA, the RCD will trip, and at this point the maximum fault voltage that can occur will be 50V. And of course the Zs is likely to be a lot lower, so the fault current will be a lot higher, and the trip time a lot quicker.

So under the 17th, with pretty much everything on a 30mA RCD, reading 1.7 ohms, or 2 ohms, or any value less than 1667 ohms is pointless because the RCD will trip long before the MCB (for earth faults)

I did a post entitled "will Zs readings be redundant under the 17th edition" or something, and we batted this one about for a while, might be worth you checking it out

 

[sparkyork]

cheers shakey ill check that post out, before i go forum surfing tho, on the new 17th certs is the max Zs box going to be 1667 then, so long as theres n RCD supplying said circuit?