Thank you for your question regarding the earthing of reinforcing mesh in a concrete building foundation. It is our pleasure to help.
While we are not too familiar with specifics of New Zealand electrical codes (AS/NZS 3000 Standard), it certainly can’t be that different from the US and British standards particularly in regards to this subject. And from what experience we have with the Australian and New Zealand Wiring Rules, bonding metallic objects is without doubt more stringent than here in America.
The short answer to your question is that, “Yes” the reinforcing mesh and/or rebar in the concrete must be bonded into the electrical earthing (grounding) system. This is what is called a “Concrete-Encase Electrode” and according the National Electrical Code it must be bonded to the earthing (grounding) system. See NFPA 70 250.52 (A) (3). We have also included a graphic for your reference.
Concrete is electrically conductive and presents a shock hazard to people. Bonding the metallic reinforcing mesh (rebar) in the concrete ensures that Over-Current Protection Devices (OCPD) such as circuit-breakers and fuses will function properly. You must also ensure that the mesh is properly tied and or welded throughout the grid so that it presents one-continuous electrical piece; this ensures that hazardous differences in potential do not form from one side of the grid to the other. Additionally, properly bonded reinforcing mesh in concrete will act as an effective earthing / grounding electrode, thereby improving the electrical system.
If the concrete foundation is not in direct contact with the earth, such as in the case where a plastic water barrier (vapor/moisture) has been placed between the concrete and the earth, you would still need to bond the metallic reinforcing mesh in the concrete. When this occurs, the code typically has requirements for installing additional earthing (grounding) electrodes.
We hope you have found this information useful. If you should have any further questions, please do not hesitate to call one of our engineers and they will happily answer your questions free of charge.
The Engineering Team at E&S Grounding Solutions
Photo credits: http://upload.wikimedia.org/wikipedia/commons/0/08/Nelson_New_Zealand.jpg