Short Circuit Test or Resistance To Earth test?

[jackecole]

Can anyone explain to me please:

What exactly is the difference between a short circuit test and a resistance to earth test ie when testing a fan / pump? If testing a componet such as a fan/pump and you are testing between the LIVE & EARTH and NEUTRAL & EARTH terminals - would a short circuit (ie fault with the pump/fan windings) be proved by the multimeter reading continutiy (ie a 0.01 ohms reading)? Is this called a resistance to earth test OR a short circuit test?

Thanks for helping.

 

[Spark123]

If you have a 0.01ohm reading between earth and live or neutral terminals when tested in isolation then the pump windings are shot at.
Ideally an insulation resistance tester would be used (500v DC for 230v AC mains accessories).
This is called an insulation resistance test.

 

[jackecole]

Thank you for your reply and help, it is much appreciated.

A short circuit test (ie done on a boiler with power off at spur and plug/wires removed) across L and N would require a reading of over 100 Ohms to be satisfactory (ie no short) - but when testing an individual component you measure between L and Earth and N and Earth instead?

A reading across L and E and N and E of 1 ohm or higher (ie no continuity) is satisfactory then?

Another question: do all multimeters advise against testing for continuity/resistance with power on, ie when testing a pump in situ? Will all multimeter fuse's blow?

 

[Spark123]

You can measure the resistance of an individual component or a set of components, I'm not sure what the minimum resistance will be as it isn't always as straight forwards as that. Where transformers and motors are involved the resistance of the coil/winding isn't directly proportional to the current it will draw.
When testing an individual item you would disconnect it and test across the L & N. Testing between L & E or N & E using a multimeter may only show dead shorts which is why an insulation tester is more suited.
Insulation testers are not to be used where there are any electronics though such as in the boiler as it can blow them.
You should never conduct a resistance test with the power on - it will probably fry the meter.

 

[jackecole]

Sorry for the confusion here!

So basically testing without power as normal but between L and N terminals is best to test for a short circuit - and a reading above continuity is good. Most pumps are about 250-300+ ohms reading between L and N.

What's the difference between a short and a dead short?

 

[Spark123]

A short circuit and a dead short are the same thing.

 

[jackecole]

Thanks for your help.

Just been looking and a reading of between L and N on a pump/fan indicates a short circuit (ie windings blown, etc)

 

[flameport]

Just been looking and a reading of between L and N on a pump/fan indicates a short circuit (ie windings blown, etc)

Not correct.

A pump or other motor will show a very low resistance between L&N, as you are measuring the resistance of the motor windings, which typically will be a few ohms.
Same applies to heating elements and incandescent lamps (light bulbs)
No reading (or infinity) would indicate the motor, element or lamp has failed.

For non-electronic items, if you get a low ohms reading between L&E or N&E, there is a fault. (The actual resistance should be in the millions of ohms range, which can't be properly measured with a normal multimeter).

None of this applies to electronic items, as the various components of the power supply will result in all kinds of results, none of which really tell you anything.

 

[onlyfitidealboilers]

expect around 160 ohms on a healthy pump, anything below 40 ohms may blow the fuse, fans are between 60 & 80 id say.

 

[ratbiker]

Sorry to resurrect this one I appreciate it is old. I am just testing my combi pump now.

Between L&N I am getting around 154 ohms which from the above post seems to be ok?

Should I be checking anything else?