electrical appliance testing?

Question

Can anybody explain the tests to be carried out and how this would be done prior to working on an appliance, eg earth continuity, correct polarity with appliance ON and OFF. testing correct voltage etc.
I have placed the probes of a multimeter on the live and neutral pins at the plug of an hairdryer but the reading stays at 1. Should I not get a resistance. It was set at 200 OHMS.
Dont worrry Im not tampering, its part of a course Im doing but curious to know more.
Thank you.

Answer 1

As mentioned above, you do not have the correct meter to do portable appliance testing (PAT). Most electrical contractors will buy meters that are specifically for this and charge anywhere from £2+ per item to test them. If you search for PAT on the Internet you will find more info.

Usually there are 3 main electrical checks
Continuity - just makes sure the item is connected so that the other readings are real
Earth Insulation Resistance - Meggar ohms reading between power connection and earth connection at 2 X voltage (500v)
Earth resistance - a separate wire clips to the item case and a current is put through the earth wire and connection and resistance measured

There are also visual checks such as plug, cable condition, fuse size etc etc

You wouldn't be able to determine correct polarity without taking the item apart and this would not generally be done

Your hair drier is most likely double insulated, indicated by a square inside another square and would have no earth connection anyway so the testing becomes essentially visual.

Answer 2

Ignore the motor in the drier. If, for round figures, the dryer is (say) 500 watts, and the supply 250v, the dryer will draw about 2A. Ohm's Law: R=V/I, so the resistance should be around 100 ohms. It may be less than this when the heating elements are cold, but should still be measurable on the 200 ohms scale. Are you sure that you have pressed the trigger on the dryer, to complete the circuit?

If the dryer has a metal case (as some professional ones do) then there should be only a minute resistance between the case and the earth pin on the plug.

The alternative version of Ohm's Law:-
An ohm is an 'ouse where a volt lives.

Answer 3

Assuming that the multimeter (which you're presently using as an ohmmeter or continuity checker) is correctly calibrated, then I'd say your hairdryer cold direct current resistance is 1 ohm. Flip the on/off switch to off and your ohm reading should be "infinite". I don't think of a polarity with AC, even tho AC plugs are refered to as polarized sometime, where the smaller blade is hot/live and the larger blade is ground/neutral.

Your hair dryer has a wattage -- usually anywhere from 400 to 1200 -- providing a convenient way to determine the current-amperage it uses AND its working resistance. Divide the wattage by the pertinent voltage to get the amperage; divide the voltage by the amperage number to get its working resistance. (example: a 1200 watt hairdryer would draw 10 amps, and have a working resistance of 12 ohms.) The working resistance (the resistance of the appliance when it's operating) may be significantly different from its cold resistance. example:

Conventional light bulbs -- if you think of them as an appliance -- have a markedly lower resistance cold than when hot. Light bulbs provide light (and LOTS of heat) because electricity is used to heat the bulb filament to white hot to emit light. (not very efficient, I add)

If an appliance has earth continuity I'm thinking that we're saying touching metallic components of the appliance with one ohmeter lead and touching a securely grounded metal component (like a water pipe) with the other lead gives you zero resistance.

When using your multimeter, esp when measuring voltages or current, always start with the highest testing range -- esp if your testing instrument has no built in overload protection. Also for yourself always think SAFETY FIRST. Enough this time.
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Answer 4

First of all you will not have a proper reading if the meter is not set to a higher Impedance value like say 2Mega ohms. If it is a plastic hair dryer them most probably it only has 2 wires and there is no earth. There should be a symbol of a box inside a box at the side of the hair dryer. That symbol means that it is Double insulated. An earthing wire has to be connected to a metal body. The rated voltage also should be written somewhere near the handle. You cannot check the polarity of the switch unless by visual inspection in this case.

DO NOT OPEN THE APPLIANCE

Answer 5

The multimeter must be set to the AC Volts position. Assuming the appliance is one designed for 120V operation you should get a reading of 120V plus/minus 10% between the “hot” lead (usually black) and the neutral wire (usually white). If there is a separate ground (usually green) the reading between black and green should be 120V, between white and green should be zero.

It is the same procedure for testing the wiring within the appliance as it is for the power receptacle.

Non-solid state components within an appliance can usually be tested for opens, shorts and grounds using the standard practices of voltage drops or an ohmmeter.

Answer 6

Multimeters are not suitable for all stages of Portable appliance testing, you need to measure insulation resistance and multimeters dont do that.

Answer 7

maybe the resistance was in the kilos