When the IET included microwave leakage testing in its Third Edition Code of Practice for the Wiring Regulations in 2008 it was based on the premise that irradiation was the primary risk, whereas the main risk from a microwave oven is food poisoning caused by inadequate heating.Continue reading
PAT Testers Blog
There has been much debating over the years about the most appropriate test current to check the integrity of the protective earthing conductor on electrical appliances.
Up until recently a higher test current of between 10A and 25A was most usual as it was thought that it was most likely to detect any damaged conductors.
However, it has been reported recently that with modern electronics this is not necessary any more - now the industry generally uses lower test currents of 100mA to 200mA.
But both the higher and lower test currents have their benefits and the IET Code of Practice recommends both 25A and 200mA.
The earth bond test is performed to check that there is a good connection between the mains and the earthed metal parts. A protective earthing conductor is there to prevent electric shock through allowing current to pass under fault conditions. Class I appliances need to have a protective earthing conductor resistance of a low enough value to prevent voltage on external metal parts becoming a level where shock is likely and to be certain that the fault current is large enough to cause the fuse to operate quickly and clear the fault.
To test this it is usual to plug the electrical equipment into the PAT tester and clip the test lead to an earth point on the appliance. The earth continuity test passes a test current along the earth cable and the tester measures the resistance. Any damage to the earth connection - the earth resistance reading will increase.
However, measurement of the protective earthing conductor is made from different components of electrical resistance which could affect the results of tests – ‘bulk’ resistance will be affected by temperature and physical pressure under certain circumstances.
Contact resistance is variable and depends on the interface between the two surfaces in contact. In reality, the contact area surface is smaller than it is thought to be, this results in the possibility of increased resistance because the current is channelled through small areas of contact. Any dirt or oxide on the surface of the material can increase the resistance at the contact.
Any of these factors can affect the results of an earth continuity test. It is important to be careful so that accurate results are achieved.
It is a common belief that higher test currents of 10A to 25A are capable of overcoming contact resistance, however large currents in the earth conductor can increase the risk of surge voltages which could damage sensitive electronic components. This is one of the reasons the 25A test is not used on IT equipment.
On the other side of this argument is that the 100mA to 200mA test may be softer and be safer for these sensitive components, but it could still have inconsistencies because of its ability to achieve a good contact point where contact resistance and contamination may result in a fail test.
This means that some test results for Class I earth continuity can produce a variable or a false fail result.
Seaward are one of the manufacturers aiming to overcome this issue by developing new technology which solves the problem. Seaward say they have incorporated their new zap test circuit technology into all their tester series models.
So, what’s your experience, do you find this a problem, is the new technology from Seaward needed?
Written by Sara Thomson
This week we’re looking at the visual inspection, not the user inspection but the Formal Visual Inspection carried out by PAT Technicians on appliances.
A Formal Visual Inspection should only be performed by a competent person. This usually means someone who has undertaken training in PAT testing and gained a certificate of competence.
When performing the Formal Visual Inspection, first the appliance must be assessed to ascertain its suitability for the environment. So, the PAT Technician should decide if the equipment being used is suitable for that job and environment.
Checks should then be carried out to identify if appliances are being used in accordance with the manufacturer’s requirements. So things like:
- Finding all cables to avoid damage
- Finding how the item disconnects or can be isolated
- Finding the ventilation and ensuring there is enough
- Ensuring all other items are out of reach or away from potential damage
- Ensuring the appliance is not in a position which stretches the cable
- Ensuring the appliance is being used with all covers and doors attached
- Ensuring there are no overloaded sockets and adaptors
- Checking that the cables aren’t a trip hazard
The PAT Technician should ensure that the user can isolate the appliance from the electrical socket in general daily use. They should also ask the user if they are aware of any faults and if the appliance works correctly.
Once this has been done, the Technician should inspect:
- The flexible cable to identify any faults or problems,
- The socket outlet
- The appliance itself to ensure there are no cracks, breaks or other problems
- The plug head to ensure there are no problems or issues with it
For equipment with a re-wriable plug facility Technicians should inspect:
- Detachable power cords for class I equipment to ensure it has a CPC
- Signs of overheating
- Cord security, polarity and connections on the internal inspection
- Cord security and burning smells for non-rewirable plugs
- The fuse size, BS mark and ASTA mark
- The plug casing and its security
- The flexible cable connection and anchorage
Once the Formal Visual Inspection is completed, the PAT Technician can move on to the electrical tests; assuming that the appliance has passed the visual inspection. If however, the appliance fails the visual inspection, there is no need to do any further tests, the appliance should get a failed label and recorded as a failed item in the logbook.
Are there any other visual checks you regularly carry out on your PAT inspections?
Written by Sara Thomson
The Lofstedt Report and the changes to PAT testing procedures which followed caused a little confusion when they were first announced. However, now these changes have been in place for a while, have they simply become a part of life or are people still finding them difficult to understand? Continue reading
Electrical Safety First recently worked alongside the Government’s ‘Fire Kills’ campaign to raise awareness of fires in dirty, messy homes in the UK. Continue reading
At the end of March the Electrical Safety Council rebranded to Electrical Safety First. Although this is now fairly well known, what is unclear is why the charity felt this rebranding was necessary. Continue reading