ZS before RCD?

[jamminuk]

Hi,

I've looked for the answer to this question on her ebut couldn't find it. Can anyone tell me why it's important to do a ZS test before an RCD test?

Thanks

Ben

 

[malcolmsanford]

You test for Earth Fault Loop Impedance to ensure you have an earth fault path.

 

[Ponty Massive]

Your Zs will ensure you meet your disconnection times.

 

[telectrix]

your Zs test proves that you have continuity between line and cpc. this should be established before RCD testing. though i am unsure if a high Zs reading would have an effect on the RCD test as such.

 

[jamminuk]

Thanks for your quick response guys, but why is it important to be done before RCD?? what could happen if you did RCD first?

 

[topquark]

The Zs reading is done first to ensure that you have a sufficiently low resistance earth fault path. If the Zs is too high, then it is possible that insufficient current would flow down the fault path to trip the RCD. In the event of a "real" fault then there may well be current flow other than down the fault path (ie through you to earth via another route(s)) so the imbalance between line/neutral would/should still cause a trip. However you need to ensure that the route you are using when testing the RCD timing is good. Hence the Zs before RCD tests.

 

[La Poste]

RCD test can last for 2 seconds, if you do not have a good earth the voltage of the earth circuit and any metal attached to it can raise to 230V for the duration of the test.
Hence it is essential to test for a good earth before an RCD test.

 

[jamminuk]

Thanks, so basically an RCD test could be dangerous if there wasn't a good earth, 1 down 200 to go ;-)