max zs and non compliance

[wrighty]

alright guys.

im just after abit of advice on where i stand with test results etc.

the current project im working on requires the installation of 10mm swa backed up by a 50a breaker.

now ive noticed alot of the jobs i do, the Ze doesnt meet the recomendations for the type of system, usually tnc-s also the maximum zs for the breaker .

these jobs are street furniture based and are usually protected by a 30mA rcd as the main incomer.

so when filling out the test sheets do i highlight this problem? or am i missing something?

my problem is ive never filled out many test sheets and im just trying to cover myself

cheers

 

[SteveEmartin]

If you are relying on the RCD to offer both fault protection and additional protection then the reading for maximum permitted Zs would be 1667 ohms according to table 41.5 - ensuring your RCD operates within the correct times for Ix and 5Ix.

If you are relying on Automatic Disconnection of supply (ADS) then you will need to make sure your readings are below the maximum readings on tables 41.3 (for BS60898)

 

[wrighty]

the problem being i cant test the RCDs as i have no permit for the 3g outage, its mobile phone equipment BTW,

so aslong as an RCD is fitted then my readings are fine?

 

[jamesBoater]

if u dont test the rcd how will you know it complies?

 

[monkeyelectric]

If the ze doesn't comply contact dno, if the zs doesn't comply thats bad design and needs sorting! You say you have no permit for a 3g outage, how do you test ze then?

 

[bigbaddave]

One way of making a curcuit comply if ZS is not achived is to fit a RCD as detailed.. However come on.. You need to ensure this works..

Personally id be inclined to look to why this was not achived first

 

[SteveEmartin]

The RCD should be tested as your initial verification - tests prior to any loads or equipment being connected. If it is condition reports you are carrying out then you will need to make sure you detail all your limitations to the tests you can carry out.

I am surprised that your Zs readings are too high - If your Ze at the origin is correct for TNCS then the design of your sub circuits should maintain a permitted Zs reading - the design may need looking at with regards to the length of run, volt drop etc..

 

[wrighty]

its not condition reports, sometimes its just minor works connecting the BTS up, as enabling works have been done and testing carried out when the enabling works are carried out, sometimes there is a certificate left in the power cabinet and it would appear reading there certificates they have had a 3g outage and have tested the RCD. other times its removing old kit and installing new, so a new circuit.

i just want to know where i stand and what i should be doing, because im not 100% happy with the situation, hence why i have come here for abit of advice not to be shot down.

the runs of the sub circuits is never more than 10m, average r1+r2 is 0.03.

i wish it was as easy as contacting DNO, it would help if i knew who the DNO for each site was.

i fully understand that the RCD needs to be tested to prove compliance.

so going on my brief description, what would you do in my shoes?

 

[SteveEmartin]

If by doing your calculations of Zs = Ze (R1+R2) and the readings you obtain are above the maximum Zs then either your Ze reading at the origin is too high and needs investigating by the supplier. Are you carrying out the tests using a non trip Earth Fault Loop Tester? as this may be the problem...

 

[wrighty]

yes i am, see ive had a few meters from the company i work for now, because the 1st seemed to give me same readings every time, and the second wouldnt test on non trip below 1ohm.

what would you suggest?

 

[SteveEmartin]

As the non trip tester uses a lower current the readings may not be as accurate or as suggested give a reading of 1 ohm - in commercial and industrial circuits when Zs readings need to be nearer 0.4 ohms or so some non trip EF loop testers struggle with this accuracy.

You can verify by calculations as suggested earlier.. this is accepted by BS7671 as long as you have noted this on your test sheet and had chance to carry out R1 + R2 or R2 using a wondering lead and knowing or inquiring Ze. Or try a good tester....

 

[Engineer54]

If as you say the Ze and/or the Zs doesn't comply, then obviously something is wrong and needs to be highlighted in any test report you complete. It's then down to your company to sort things out. Be it contacting the DNO or revising the design of the that particular installation.