Product category: Compliance Engineering
News Release from: Clare Instruments | Subject: Earth bond testing
Edited by the Electronicstalk Editorial Team on 23 April 2007

The importance of earth bond testing

John Barnett details new developments in earth bond testing and explains its role as part of the mandatory requirement for electrical safety testing of products in a production environment

The earth or ground bond test is also referred to as earth continuity, earth grounding or protective bonding test and is quoted throughout National, European and International product standards. The Low Voltage Directive (CE Mark) and product approval agencies stipulate that this test must be performed on 100% of all Class I earthed, grounded, or metal clad products produced during manufacture as part of the criteria for routine testing.

The ground bond test performs a measurement of resistance to determine that the equipment remains safe in the event of internal live conductors touching the external case of the device under test.

The test ensures that the earth or ground conductor can carry sufficient current to enable the supply protective fuse in the plug top, or circuit breaker, to blow before the case rises to a hazardous voltage.

A low voltage high current test is used to ensure that earth connections are correctly made.

A failure indicates loose or poor earthing/ground connections within the device under test.

The ground bond test is applied to Class I, earthed/grounded or metal clad equipment only and cannot be used on Class II equipment where protection is provided by double insulation.

A low voltage (typically less than 12V AC) is introduced to the input earth connection, including the supply cable, and a measurement is then taken using a clip or probe connected to all external metal surfaces.

The test is applied without the equipment connected to the mains supply.

The test is carried out with the power-input lead that will be issued with the equipment.

It is not satisfactory to use a 'common' or 'sample' lead for all tests.

Test current varies between international standards, although it is commonly either 10 or 25A AC, depending on the power rating for the product under test.

As this test is making a low resistance measurement, Kelvin or four-wire techniques are desirable as they remove the effect of the test lead resistance, and ensure that repeatable results are achieved.

The test provides a measurement of resistance and the pass-fail criteria varies slightly between standards, although the majority of product standards are becoming internationally harmonised.

In general terms a test using 25A requires the product earth or ground resistance to be less than 0.1ohm.

In contrast lighting or luminaire equipment, which is tested using 10A, requires a pass/fail threshold of less than 0.5ohm - this is because the typical supply current would be less than 3A and the earth conductor has a smaller cross sectional area.

Machinery constructed to EN60204 requires a measurement of voltage drop and the permissible drop is determined by the cross sectional area of the earth/ground conductor.

Transferring the type test to the production environment can cause problems as type tests often prove impractical in terms of time or methodology.

International standards and approvals agencies have taken this into account and offer methods for applying a 'routine' earth/ground bond test to suit the manufacturing environment.

The routine earth/ground bond test can usually be performed in 2s, whereas the type test can require in excess of 1min.

Clever test application techniques have also been developed with manufacturing in mind, such as the use of no-burn test probes, which employ a microswitch to trigger the test current, and integral pass LEDs.

These probes are pressure activated by a microswitch giving a unique 'no burn' feature, which means no burn mark is left on the equipment as result of applying the high current.

This type of probe also features an LED to indicate the test status during the test sequence, when testing products that prevent the operator from viewing the tester.

The integrity of the test relies on all individual external metal surfaces being tested.

On the production line it is necessary that operators are familiar with the relevant test points and identify primary earth points so that fast, accurate testing is ensured.

Test equipment should be used with easily visible pass indication.

Some equipment have difficult access earth points, for example the element in a kettle is the only earthed component, and other equipment have earth points that are hard to access because of plastic shrouds etc.

In some cases these difficulties may be addressed during the design stage, so that proper consideration is given to the positioning of earth test points and the test process requirements during production.

In others the only option is design-dedicated bespoke test probes.

The causes of earth/ground bond failure come under two main categories: manufacturing errors and design shortcomings.

Loose or painted terminals are common causes, as are earth leads which have been accidentally cut or broken.

The failure to connect earth/ground straps during build or inspection can cause problems.

In these cases a manufacturer's procedures should be able to address the majority of issues and it is worth mentioning that many of these errors could be discovered prior to the earth/ground bond test by a proper process of visual inspection - either prior to test or at relevant points during the production process.

In terms of design related causes of failure, the earth/ground bond test should include the power input lead.

As the earth/ground bond measurement of resistance is dependent on the length of the earth conductor as well as the cross sectional area, an excessively long power supply lead may give a failure reading.

Leads of 2-3m length will not cause any problems in achieving the normal pass/fail criteria, but products such as floor cleaners which have much longer supply leads, due to the nature of their use, will require careful assessment of their test results.

It is important to consider also how manufacturing tolerances in production can affect the reliability of the product earthing/grounding.

For example, joints that rely on compression may not perform as anticipated during design and hinges may not provide the level of earth/ground bonding as first intended.

It is therefore imperative that during manufacture 100% of product is thoroughly tested. Request a free brochure from Clare Instruments....

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