Testing for continuity with Voltage on contact

[dbh6]

I don't know but i remember someone telling me that say for example you have a contactor or a relay generally if the coil is energized the corresponding contact will either close if they are NO or open if they are NC. Now in the instance where you energize the coil, you should have continuity between the NO contact terminals this is when their is no power flowing through the contacts. So say you do the same test but with either 24vdc or 120vac passing through the contacts and test for continuity will that cause any issues???? Generally when energized with power flowing through i test for voltage but want to know if the continuity test would be ok as well

 

[Mark Snodgrass]

Only if you don't mind blowing up your meter. If for some reason the contacts are actually open (ie. burnt up), then you will be putting the voltage across your test leads. Some multimeters have fuse safety for such an error, but speaking from experience you dont want to find out the hard way yours is not. The best way to check when the contacts are energized is with the voltage setting, check voltage on both sides to ground, or neutral (common). Or you can check your voltage drop across the contacts, there should be a miniscule drop across a set of good closed contacts.

Be Safe
Mark Snodgrass

 

[dbh6]

thanks

EDIT** i have actually test for continuity with voltage on and it actually tested and beeped signaling their was continuity i guess my meter is fused, although i did hear a little chatter in the meter.

 

[Lancie1]

It actually tested and beeped signaling there was continuity...

You were lucky that the contact was closed. If it had been open with the power on, like Mark said you would have blown the fuse in a good meter, or burned up a cheap unfused one. Most of us have destroyed at least one meter by checking a contact without first looking to see if the big knob was turned from "Ohms" to "AC Volts".

For a closed contact, both sides are at the same voltage potential. When you put a meter lead on each side, it is like you put both leads on the same electrical point and with no potential difference. No current can flow at 0 volts potential difference, so the contacts simply act as a conductor for your Ohms setting through which to pass its internal current.

On the other hand if the contact is open, your meter becomes the resistance path to the lower voltage on the other side of the contact (probably through a coil in a relay, starter, or solenoid). This will burn out the sensitive current sensor meter circuit, unless a fuse blows first.

 

[mendonsy]

If you MUST test for continuity, test for voltage across the contacts first. If there is voltage across the contacts, you don't have continuity. If there is no voltage present then it is safe to do a continuity test.

 

[TConnolly]

A voltmeter reads voltage drop, not absolute voltage.

To check a closed energized contact set the meter to read voltage. Assuming that the downstream line has continuity then you should read 0V or at most a few millivolts drop across the contact when it is closed. If you read line voltage across the contacts then the contacts are not closed.

If you have doubts about whether the downstream line has continuity then do as Mark suggested, and read the voltage drop to some known reference such as neutral or ground, or if a three phase system then with respect to another leg.


You read a fuse the same way, if you read line voltage across a fuse then you are dropping line voltage across the fuse so it is blown.

 

[dbh6]

guys i appreciate the info, i just wanted to know what issues i would face if i tested for continuity without having the meter setting to measure voltage drop, i had it on a continuity test, through which mark had described the consequences if you do that test while the contact was open.