How do you make sure a piece of electrical equipment is safe?

Just like in everything we do, there is a right and wrong way to test appliances. You can get 'real', straight-forward and easy to read information from the Standard AS/NZS3760:2010 which covers not only how to test but also the environment for frequency of inspection and testing which is now based around usage rather than being specific site based. Testing can be quick and easy if you have good equipment and the right training.

What tests need to be done?

The correct tests need to be carried out on your Class I or Class II appliance in the following order:

Firstly, before any testing can be carried out, decide what type of equipment it is eg CLASS I (earthed) or CLASS II (double insulated).

Let's look at what these words (together with a few other common terms) mean:

Class I Equipment
Safety Class I electrical equipment is electrical equipment that relies on the Protective Earth and basic or single insulation for its safety - such items usually have quite a lot of exposed metal and are often quite heavy, although this is not always the case. Some examples of Class I equipment could be dishwashers/washing machines/irons/fridges/ electrical machinery/drill presses/saw benches/bench grinder etc.
  • This type of appliance is often referred to as an 'earthed' appliance
Class II Equipment
Safety Class II electrical equipment relies on two layers of insulation, many times reinforced insulation will replace the two layers of insulation, but the reinforced insulation will be equivalent to the two layers of insulation.
  • This type of appliance is often referred to as a 'double insulated' appliance
  • It will normally be marked with the doublebox symbol
 
RCD (Residual Current Device)
An RCD is used for increasing electrical safety and is designed so that it will disconnect the power when the residual current reaches or nears a preset value. For RCDs designed to protect people this figure must be set at 30 milliamps or less.
Isolation Transformer
This is used for increasing electrical safety and is designed so that the two sets of windings (the primary and secondary) are separated from each other by at least the equivalent of that required to achieve double insulation, thereby ensuring that the electricity from the secondary (output) side is isolated from earth.
EPOD (Electrical Portable Outlet Device)
This is really a multiple outlet device and will commonly be called a multibox in New Zealand or a power board in Australia.

How to carry out the testing:

The Visual Inspection (this test is carried out on all types of equipment)

This is very important as approximately 90% of faults can be found by this inspection alone.The Visual Inspection is not just an inspection, it basically is'everything but'what is done during the actual electrical testing process. This part of the testing process may involve:

  • Smelling
  • Pulling on leads
  • Tapping the appliance
  • Shaking
  • Turning some part/s
  • Checking by trying to see that retractable guards actually retract... and all manner of other things may be done to ensure that it is safe to operate.

Then depending on whether the equipment is Class I or Class II the following tests are done:

For Class I (earthed) equipment
  • Earth Continuity Test - this may be done with an Ohm Meter or a Portable Appliance Tester (PAT). The Standard AS/NZS3760: 2010 suggests a test current for PATs of 100mA to 25 Amps.
  • Insulation Test/Leakage Test - a leakage test shall be done on appliances that have internal switching that is only activated when the equipment under test is powered at operating voltage. For equipment that does not have internal switching as above then a 500 Volt insulation test is an acceptable alternative.
For Class II (double insulated) equipment
  • Insulation Test/Leakage Test - a leakage test shall be done on appliances that have internal switching that is only activated when the equipment under test is powered at operating voltage. For equipment that does not have internal switching as above then a 500 Volt insulation test is an acceptable alternative.
For Cord Extension Sets
Cord extension sets with re-wireable plugs fitted need to also have, in addition to the above tests, a
  • Polarity Check
For RCDs
  • Earth Continuity Test
  • Insulation Test/Leakage Test
  • Trip Time Test - in addition to the above tests RCDs shall have a trip time test performed on them at the time period specified in Table 4 AS/NZS3760: 2010

Tests required in more detail:

EARTH TESTING

Legally it is a requirement - but is it really necessary?

Potentially the most dangerous appliances are Class I appliances (earthed appliances) eg microwave ovens/bench grinders and the like, but also in this category are extension leads. Class I appliances are designed to have an earth - this means that in the example of the floor polisher the body of the polisher is connected to earth - literally to the ground via an earth conductor which goes right back to the building switchboard and then into the ground the building is sitting on. If this conductor is damaged anywhere then the consequences can be fatal - this has been explained in greater detail below. It is obvious therefore that the conductor needs to be tested.

The difference between using a Multimeter and a PAT Tester with a high current earth bond capability is that a Multimeter does a Continuity Test where as a PAT Tester does an Earth Bond or Earth Integrity Test.

There is a VERY BIG DIFFERENCE between the two tests.

How can this be done properly?

Unfortunately, most people believe they are carrying out the testing properly if they are using a Multimeter or an Insulation Tester, but these types of instruments will not usually pick up an earth fault unless there is a complete break in the conductor.

There is another type of tester available on the market that people, including many electricians, may not be aware of - it is called a Portable Appliance Tester or 'PAT' for short. The PAT is specifically designed to test electrical safety and does all the different tests necessary to ensure the electrical safety of the appliance and makes testing easy.

It is very important to first ensure that the Earth is satisfactory before undertaking the Insulation Resistance Test

To see the difference between testing with a Multimeter or an Insulation Tester compared with a suitable Portable Appliance Tester (PAT), try doing this simple test:

Attach a 10 Amp fuse across the leads of a
Multimeter with the Multimeter set
on Ohms range
Attach a 10 Amp fuse across a battery charger

In these two diagrams the battery charger represents a PAT Tester, something that produces a low voltage high current electrical supply. The 10 amp fuse in both cases represents the last remaining strand of a multi-core earth wire.

What does the above diagram show?

If you test with a Multimeter or any type of Continuity Meter/Buzzer or Insulation Tester you will see that the meter indicates that the earth conductor is OK and therefore the earth conductor is presumed to be safe - however sadly this is not the case. Now try the same test with a battery charger - the fuse will blow. Now you know that your earth is faulty and you can fix it.

In recent years it has become common for PAT Testers to be small and sometimes a lot lighter as they have less inside! Many PAT Testers nowadays do not have the ability to test an Earth by applying 10 Amps or 25 Amps through the protective earth conductor - it is our belief that this is a step backwards as far at PAT Testers, and safety, are concerned.

Size and price versus functionality is what the compromise has become.

There are still many (usually, but not always) more expensive PAT Tester that are able to do the 10/25 Amp Earth Bond Test - these we would define as electrical safety testers compared to electrical compliance testers. Both can meet the requirements of AS/NZS3760, but one goes a step further and is more likely to fail appliances with faulty earths.

Doing safety testing with just a Multimeter or similar type of instrument is like carrying an umbrella around in case it rains; only to find when it does rain that the umbrella leaks! An earth conductor substantially damaged or not there means that in a situation where a fault occurs the operator becomes the earth ie the operator becomes the means by which the active 230 Volts supply returns to earth - likely killing the person in the process!! It is essential that when an earth conductor is needed that it is able to pass the earth current required otherwise you or someone else will become the earth.

You should by now be able to see the difference between an Earth Bond and Earth Continuity Test ie an earth bond test shows whether the earth conductor is in a satisfactory condition; whereas an Earth Continuity Test (100 milliamps or less) using Multimeters and the like will possibly only show the earth is connected, not its' condition.

RESISTANCE TESTING

Insulation Resistance Tests must not be carried out using Ohmmeters or Multimeters because these meters only produce a small battery voltage in the region of 1.5 to 9 volts. This small voltage is insufficient to pressure test the insulation to expose any weakness in it and therefore is totally inappropriate for Insulation Resistance Tests.

Insulation Resistance Testers and PAT Testers on the other hand are specifically designed to produce 500 Volts (or even greater voltage values for some applications) which places the insulation under stress. This test will indicate any weakness that may break down under normal use on the 230 Volt supply. Itis a requirement that the resistance measured by the Insulation Tester in this way should be not less than 1 Megohm (1,000,000 ohms) for an electrical appliance to pass this test.

A practical example showing how a Multimeter or Ohmmeter is inappropriate to test the insulation resistance of an electrical appliance is detailed by the following example:

A 230 Volt floor polisher has a partial insulation breakdown between the motor windings and the earthed metal case. The polisher when in use and at its normal operating temperature, causes the sub-circuit protective fuse to blow as a result of the insulation breaking down to the earthed metal frame.

The polisher, when tested with an Ohmmeter or Multimeter, does not indicate the presence of a fault, as the meter voltages are so low that they do not stress the insulation and therefore do not reproduce the breakdown that occurs when the appliance is in use.

However, testing the same polisher with an Insulation Resistance Tester producing an output of 500 Volts causes the insulation to break down under test, thereby indicating a low insulation resistance value and the presence of a fault path.

NOTE:The importance of a sound low resistance earth continuity conductor is paramount to the safety of any electrical appliance that requires an earth. A test as described above would not achieve anything if the integrity of the earth were not first proven to be in good condition. Should the appliance have an open circuit earth continuity conductor, it would produce a high resistance test result, but in fact when the appliance was plugged in it would enliven the case and expose a potential shock hazard.

Alternative Leakage Test as required AS/NZS3760:2010

LEAKAGE TESTING

This test was first introduced in the 2003 version of the Standard and it can be used to replace the 500 Volt Insulation Test.

The advantage a Leakage Test has over and above the 500 Volt Insulation Test is that the Leakage Test when carried out on an appliance at operating voltage ensures the whole of the appliances' circuitry is fully tested. The old 500 Volt Insulation Test would not fully test many appliances ie those with internal switching that required operating voltage to activate switches. Hence an appliancemay test safe and not be, when tested with a 500 Volt Insulation Test.

Some fatalities could have been prevented if a Leakage Test had been done instead of the 500 Volt Insulation Test.

NOTE:A 500 Volt insulation test is an adequate test providing all of the appliances' circuitry can be tested using thismethod.

RCD USE & TESTING

Unfortunately we can all be careless around electricity -RCDs can give limited protection against a fatal shock

An RCD is specifically designed to minimise the risk of a fatal electric shock when working with electricity. It is an electronicsensing device which constantly monitors the balance of the current flow in the live and neutral conductors to ensure thatshould even a minor leakage occur the RCD will switch off the power within 300 milliseconds (for personal protection RCDs)iein less than the time of a single heartbeat -which could mean the difference between life and death.

You need to remember the following:

  1. When properly connected an electric current flows through the live wire on its way to the appliance and returns to ground via a neutral wire
  2. If for some reason the current 'leaks' (through dampness, frayed wiring, faulty connections etc) the current will take the most direct path to earth - often through a persons' body. In this situation if the power is not disconnected instantly then there is a strong possibility thatthe person will die.
  3. The path taken by the current also contributes to the severity of the electric shock. An electric shock which flows from the hand through the arms and to the chest will affect the breathing and heartbeat.
  4. The greatest contributing factor to the severity of an electric shock is the exposure time to the current. An RCD cuts off the power in less than the time of a single heartbeat

Testing an RCD can be done in 2 ways:

Ethos 7100 RCD Tester

1. A push button test - this requires no test instrument.

2. By the use of an RCD Tester an RCD Tester is designed to check the trip current and trip time. Most RCD Testers can measure from 1 millisecond to 2000 milliseconds. Obviously this trip time test cannot be done Many RCD Testers will be capable of conducting both an AC and DC test, however in New Zealand & Australia to meet the requirements of AS/NZS3760 no DC test is required.

Sonel MRP 201 RCD