Measuring Zdb

[Paul.M]

Spearmint your just a little muddle up thats all, you are almost there, just need to clear a few thing up with in the morning. too late now I'm off to bed but if you are loosing sleep over this, dont! It will all be clear this time tomorrow (having a day out with family, back on in the evening).

 

[Spearmint]

Yeah no worries, certainly not loosing any sleep over it. Maybe I'm going around in circles with this but to measure the Zs of this particular RFC it has to be energised, and for that to happen DB1 must also be energised. Both DBs are covered by RCDs so using the plug top adapter and setting the meter to the relevant no trip current (30mA in this case) is the only way I can think of doing it. Obviously you can use probes into the back of the socket but this would yield the same results surely?

How else can it be measured under live conditions?

Sigh, I'll forget about it for a bit lol. Have a good day all!!

 

[Simonslimline]

When taking a Zs you need a complete circuit (Loop) down the CPC, back down the main earth, back to the transformer and then back through the phase conductor to the point of the fault. Have you tried any other sockets? Their might just be a high impedance on the circuit mate which the tester is picking up. When you do a loop test you can either do high loop which sends 25Amps around the circuit, or a no trip which will send 15milli amps down the circuit. A 30ma RCD can trip anywhere between 16ma and 30ma and comply I believe. I would try taking a test from other sockets on that circuit and also at the DB.


These loop test currents may vary with different testers. The values i have used are for a Megger 1552/3.

 

[Spearmint]

When taking a Zs you need a complete circuit (Loop) down the CPC, back down the main earth, back to the transformer and then back through the phase conductor to the point of the fault. Have you tried any other sockets? Their might just be a high impedance on the circuit mate which the tester is picking up. When you do a loop test you can either do high loop which sends 25Amps around the circuit, or a no trip which will send 15milli amps down the circuit. A 30ma RCD can trip anywhere between 16ma and 30ma and comply I believe. I would try taking a test from other sockets on that circuit and also at the DB.


These loop test currents may vary with different testers. The values i have used are for a Megger 1552/3.

Tried it on all twelve socket outlets, all give the same reading within 0.2ish of each other. It's a really bad first example in a way because I've yet to carry these out on the course but on site ended up with silly readings. I'll be taking a Ze next time I'm there but that isn't going to change the measured Zs from the sockets which are high. It isn't even exclusive to the one circuit either, there's two separate RFC from the new board and they also have the same high readings.

 

[Simonslimline]

Like you said, do a Ze when you go back and establish that it is within 0.35 ohms(TN-c-s) if you have a good Ze the you know the problem is within the installation, just a process of breaking it down until you find the cause of the high reading.

Its all good practice and experience

 

[Spearmint]

Like you said, do a Ze when you go back and establish that it is within 0.35 ohms(TN-c-s) if you have a good Ze the you know the problem is within the installation, just a process of breaking it down until you find the cause of the high reading.

Its all good practice and experience

Will do. I fear this could open a can of worms though... main board has been in for years, if the issue turns out to be from the main board then... ah fun times lie ahead!