Ground Bond Testing
Protective ground conductors are designed to allow a safe and easy path (low resistance) for electrical leakage and fault currents to flow, which allows the protective fuses or line current monitors (RCD’s, GFI’s) to operate and interrupt the supply voltage. This provides an important means to reduce the risk of injury by electric shock and also stops the release of energy which may ultimately lead to fires. In class I electrical equipment the protective ground conductor resistance needs to be of sufficiently low value, in order to prevent the voltage on external metal parts rising to a level where the shock potential presents a hazard to life.
Although many class I medical devices are supplied with a ground reference point, most, if not all, medical devices require multiple ground bond tests to validate the connections of additional metal accessible parts on the enclosure.
A test current is applied between the ground pin of the mains supply plug and any accessible metal part (including the ground reference point) via a dedicated ground bond test lead (clip/probe).
Figure 7 shows a representation of the ground bond test.
Figure 7 - Ground bond test in class I equipment
For fixed installations a point-to-point continuity measurement can be made by fitting a second lead into the aux ground socket. The resistance is then measured between the two leads.
The IEC 62353 requires a minimum test current of 200mA, either AC or DC. When using a DC test current, the resistance must be tested in both polarities of the test current. The highest reading will determine the pass or fail result of this test.
The open circuit voltage of the current source should not exceed 24V.
The test limits in IEC 62353 are set to:
100mΩ For a detachable power cable up to 3 meters
300mΩ For a class I device including power cable (not exceeding 3 meters)
500mΩ For a medical system consisting of several medical and non-medical pieces of equipment. See definition of a medical system in IEC 60601-1:2005
Ground Bond Test Considerations
Checking the protective ground during routine testing is different from that undertaken during the type test approval. While testing at the design stage highlights the capacity of the design to cope with fault currents, the quality of the protective ground is more important during routine testing. It’s important to remember that contact resistance can be easily overlooked when using the required 25A in IEC 60601 because high test currents can temporarily repair poor mechanical contacts.
Contact resistance is made up of two components:
- Restriction resistance (where the conductive cross section is reduced)
- Film resistance (the possible resistive layer between the two conductive surfaces due to film oxidation, dust etc.)
Lower test currents, typically less than 8A RMS, are unable to temporarily overcome contact resistance (both film and restriction resistance) and thus highlight problems as a result of aging (increased restriction resistance due to softening of spring loaded force on the contacts typically found in removable power cords, see figure 8).
Figure 8: Example of increased contact resistance in spring loaded contacts
High test currents (10A or more) tend to provide a more constant reading (high precision) even if there is a potential constriction in the protective ground path. High test currents might also be destructive to parts of the DUT which are connected to the protective ground but have a functional purpose (e.g. screening).
Therefore, IEC 62353 recommends that protective ground connections are tested with a 200mA test current to highlight aging power cords although high readings could be as a result of film resistance which can be removed.
Combining a high pre-pulse (to clean the film resistance) and measuring with a low current to show up any restriction resistance, is the most accurate way to determine the quality of the protective ground path.
Precision vs Accuracy
When performing a ground bond test, remember that accuracy must take precedence over precision as having a consistently wrong measurement is precise but not very accurate. Using a high test current might provide a higher precision (see figure 10), but would not necessarily give you a more accurate representation of the quality of the protective ground circuit due to its capability to temporarily repair constriction resistance. Lower currents are not able to provide a false positive and are therefore fail-safe.
Low test current only (see figure 9) - Possible low accuracy and low precision as high readings could be due to film or constriction resistance.
High test current only (see figure 10) - Possible high precision but low accuracy as aging cables with poor contact resistance will give the same readings as a brand new good cable.
Low test current with high current pre-test (see figure 11) - Cleans film resistance, any high readings would be down to poor contact resistance thus high accuracy and high precision.
A separate white paper on high vs low test currents is available free of charge on Rigel Medical’s website at www.rigelmedical.com/rigel-downloads