CF Electrical Services NW

Testing an item

Testing falls into two catergories : Visual Inspection and Electrical testing

Before a item can undergo any electrical testing it must first pass a visual inspection this includes checking the following for

• any damage to the casing and flex
• correct fuse rating
• making sure the plug is wired correctly including poarity, termination lengths, secure fixing, water ingress or overheating

When an item has passed the above it may then be electrically tested. There are several types of electrical tesing and it is important to use the correct type of test for that particular item. An example of this would be a computer. A computer is made up of several items to test each part may need testing differently. Let me explain taking int account the seperate item under test would be IEC mains leads, the computer tower, and a monitor.

An IEC mains lead usually has a fixed plug you would therefore not be able to check the inside as it is sealed. You would test this for Earth Continuity, Insulation resistance at 500v ac and polarity.

The computer tower would then need the IEC mains pluging in again the computer tower is usuallly a class 1 piece of equipment (equipment which is protection of electric shock does not rely on basic protection only but includes means of connection of any exposed parts to a protective conductor in the fixed wiring installation which should carry any fault back to the fuse and disconect) Therefore we need to test that so we test the earth continuity again, when we come to test the insulation because computers have delicate electronics inside it is generally accepted that you would then test the insulation with 500v ac instead you would lower that amount to 250v ac as not to impair the instrument. You may also want to test the load of the item this is where the test machine will switch on the equipment under test and check the current being drawn is not more than it should be.

A screen monitor or television maybe a class 2 item (this is where the basic insulation has an an additional supplementary insulation or reinforced insulation and does not rely on the Earth to carry the fault) the symbol for a class 2 piece is a square within a square and is usually found on a sticker or embossed into the plastic casing of an item. It is therefore impractical to test for Earth continuity and only insulation and possible a load test need apply.

Types of Testing

The Earth Continuity Test which can only be applied to class1 equipment or cords that is equipment that relies on a connection with Earth for it's safety (protestive earthing) and/or needs a connection with Earth for it to work (Functional earthing).

Hard test. A continuity test should be made between the Earth pin on the plug and accessible earthed metal parts of the equipment. A test current of no less than 1.5 the rating the fuse and no greater than 26A fo ra period of between 5 & 20 seconds.

Soft test. test the same as above only the short circuit test current should be in th erange of 20mA to 200mA.

The measured resistance should not be more greater than 0.1ohm plus the resistive length of supply cord.

The Insulation Resistance Test is normally measured by applying a test voltage of 500v dc although this may not always be suitable where intricate electronics have been used as this voltage may deteriate the reliability of such equipment ie. IT or mailing equipment. A so called sft test using reduced voltage of 250v dc is preferable. The test is made by connecting the live conductors (phase and neutral) together inside test equipment it should be capable of maintaining a test voltage with a resistive load of 0.5mOhm. A class1 insulation resistance greater than 1.0mOhm whilst class2 should be 2.0mOhm

Class2 equipment the test probe should be placed against any metal casing as it has no earth pin.

Protective Conductor/Touch Current this is an additonal or complementary test to the insulation test an altenative for IT equipment. An insulation test should be carried out before any powered test as this may detect any hazards such as loss of insulation. The Protective Earth/Touch Current is measured from internal live partsto Earth for Class1 equipment or from internal parts to accessible surfaces on class2 equipment. The test voltage is the supply voltage. Caution should be taken with rotating mechanical machinery etc. The list below gives the values that should not be exeeded under a 5 second test.

Portable or Hand Held Class1 equipment 0.75mA

Class1 heating appliances 0.75mA per kW a maximum of 5mA

Class1 not in the above descriptions 3.5mA

Class2 0.25mA

Functional Test this is a simply test to make sure the item switches on and off correctly.

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