Zs reading high!

sjm · Feb 25, 2012

Hi guys, what do you think of this?

Ze reading of 0.13. Zs reading at the CU of 0.12 with bonding attached etc.
Shower supply R1 + R2 reading of 0.16. Zs reading at the Shower 0.91.
Using my new Megger LRCD220 meter on 2 lead Hi setting for Ze and Zs at the board.
Then using it on No trip setting for Zs reading at Shower.
Can anyone think of a reason for the incorrect reading, or is the meter US as I think.

Thanks in advance.
Steve.

spark 68 · Feb 26, 2012

It is acceptable to put a measured Ze + a measured (R1+R2) and a calculated Zs on your sheet, what is not acceptable is to use a 'ball park' Ze (not measured, or obtained by enquiry) and calculating your Zs from this.

Like others my 'No trip' readings are always slightly higher, but I still take the reading to make sure it is close to my calculated/expected Zs (see above), but I don't record this, I do as in the previous paragraph above (calculated.)

sambotc · Feb 26, 2012

Thanks sparks68 that seems to be the most logical way around the problem and it is what I have done in the past. I always take a measured Ze even though most of the documentation seems to state Ze (by enquiry) which seems odd and from reading previous posts on here, most likely impossible!

Why you would not measure Ze I don't know, it would surely take longer to phone up than to disconnect the main earth and attach the test leads?

needasparks · Feb 26, 2012

sambotc said:
Why you would not measure Ze I don't know, it would surely take longer to phone up than to disconnect the main earth and attach the test leads?

What about on an installation that cannot be completely turned off, An Airport or even an office block ?

sambotc · Feb 26, 2012

Never been in that situation so yeah I can see there being times where it is not practical to do so, but even so that would normally be in more exceptional circumstances from the average sparks I would guess? Still I can't see any reason for documentation refering to Ze by enquiry as if it is the prefered option?

Or am I reading into it wrong?

needasparks · Feb 26, 2012

Most certs i think are designed for all situations and there is always the chance one turns up one day, but your right domestic has no reason at all and it is rare other than ind/com.
edit
should say, remeber certs are based on models shown in 7671 so i doubt they can deviate

mrloy99 · Feb 26, 2012

Its quite easy, i believe, to check your resistance values without spending a fortune on expensive test-check boxes.You can buy resistors for a matter of pence from Maplins get a selection high & low values.