×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

RCD and earth conductor sizing

RCD and earth conductor sizing

RCD and earth conductor sizing

(OP)
Hi All,

I was once told during a course that if you had high earth loop impedance, instead of increasing cable sizes, you could use an RCD.  I can't find the reference in AS/NZS 3000 (Australian Wiring Rules) to say this is OK?  Does anyone know where to find it?  Happy to get opinions on other IEC based wiring rules - they are usually similar.

If this is OK to do this, what size does the earth conductor now need to be?  I guess you might not technically need it anymore?  Would you just select the size as per the usual tables and/or size it to thermal constraints? - it obviously wouldn't be large enough to reduce fault loop impedance - hence the RCD?

Thanks.


 

RE: RCD and earth conductor sizing

If RCD means "Residual Current Device" this is a good protection device against electric shocks. But the grounding it is a general protection against electric shock and fire protection also by forcing the opening of a breaker protecting the circuit.
RCD will work also if the grounding connection will show a limited resistance to ground. If the minimum current to be sense by RCD could be 10mA a maximum resistance of VL_G/0.01 ohm has to be provided. A good security factor is 10 then R<=VL-G/0.1 will be better.
You have to check the RCD by pushing the disconnecting button at least once in a half of a year. The time to clear the fault is very critical mainly in the electric shock case as the human body has a limited time to survive under the certain potential rise.
Conclusion: the use of RCD is very indicating, but a good grounding it is compulsory.
 

RE: RCD and earth conductor sizing

healyx, check out section B4 of As3000 - 2007 and have a read.

I guess you might not technically need it anymore?  

You'd be guessing wrong. The earth wire in a TN-S system, the primary distribution method in Australia, is a safety earth. You don't go willy nilly removing equipment that is installed for safety reasons :)

Also, all conductors – line, neutral and protective – must meet the size requirements of the adiabatic equation. Clause 5.3.3.1.2 of AS 3000 in conjunction with Section 5.3 of AS 3008.1.

RE: RCD and earth conductor sizing

(OP)
Thanks guys.

Sibeen, I am familiar with that section of the standard but it doesn't really answer my questions which are:

(1) If you can't economically achieve a low enough fault loop impedance with cables, is it OK to accept that and use an RCD?
(2) If you use an RCD in this case, what is the minimum size of the earth cable now?  Is it just found via the adiabatic equation?

I would never remove the earth cable, I was just making the point that if you use an RCD, the earth cable isn't serving the purpose of providing a low enough impedance to trip the circuit breaker on an earth fault.  Disconnection is now determined by the current sums of the active and neutral cables.

 

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources