Ground bonding

The Ground Bond test
The Ground Bond test, also referred to as Earth Circuit Continuity, Earth Grounding or Protective Bonding, is a common requirement in National, European and International standards relating to Class 1 electrical products. As one of a series of "core" electrical safety tests the Earth Bond test is designed to verify the integrity of the protective conductor (earth wire) and that all parts of the conductive case of such an ‘earthed’ product are properly connected to the Earth terminal.

Verification is carried out by measuring the resistance of the earth path at a voltage of less than 12V and with a current of at least 1.5 times the design current of the product (but not exceeding 25A). Most standards require that this resistance should not exceed 0.1 ohms, to ensure that sufficient current will flow through the earth path, in the event of internal live conductors coming into contact with the case, thus causing supply circuit fuses to rupture. This provides protection for the user.

For products that have low current ratings, such as lighting equipment and some audio/visual products, and in particular where the cross sectional area of the protective conductor is small, a resistance not exceeding 0.5 ohms may be acceptable. Similarly, lead length may give rise to high resistance readings, which again may be acceptable under specific requirements relating to the product.
The Machinery Directive (EN 60204) recognises that varying sizes of protective conductor can be used, and addresses this by requiring that the test is applied at a constant current of 10A. With the resultant volt drop conforming to set levels that are dependent on the cross sectional area of the protective conductor.
Functional earthing (as opposed to safety earthing) may be found on some electronic equipment where the only connection to earth is via printed circuit board tracks and is primarily intended for EMC screening purposes (e.g. the metal shells of signal connectors on IT equipment). In these cases the high current tests described above may cause damage and a lower test current, typically 100 milliamps, may be used.

Common faults
The causes of typical earth bond failure include - loose terminals, paint around connection points being poorly cleaned off, earth wires accidentally cut or broken during assembly, missing or unterminated wires, etc. Most of these will be occasional faults but where they become persistent, investigation and subsequent improvement to manufacturing procedures should address the problem.