Hi,

I need some clarifications on measuring continuity in control circuit when it is still live due to checking faults while not affecting the power supply.
To my knowledge, when you measure a voltage drop in between an open switch before a coil which then connects back to the neutral which gives a complete path, it will give you a voltage drop. Lets say the voltage drop given is 120V.

If you were to measure the continuity in between the coil while the switch is closed, there will be a potential difference in voltage and will result in fuse blow of the meter.

What I know is if there are no current that is going back to the neutral. It is safe to measure continuity between the open switch.

Here are my questions,

1) When the switch is close, the coil will energise and current will go back to the neutral. Is it safe to measure the continuity between the closed switch as it gives no potential difference? (No potential difference meaning the wire measured has no voltage drop and is live with 120V).

2) When the switch is open, there will be a voltage drop of 120V in between the opened switch. Is it safe to measure the continuity of the opened switch as there is voltage drop of 120V? By doing this, will my multimeter be shorted by the high voltage/current and allowing it to make a complete path and energise the coil? (Note that while the switch is open, the coil is de-energise).

Thank You.

 

You cannot, and for your own, and the meter's safety, should not measure continuity on a live circuit.
What you can do is to measure the voltage across the closed contacts or between the two ends of a wire. If the contacts are working well and have low resistance then the voltage you measure will be very close to zero and likewise across a length of wire.

 

The way to verify continuity on a live circuit is to measure the voltage between the ends of whatever is being checked. First have the meter set at the circuit working voltage, if no voltage is read then switch to a lower voltage range. Of course it is important to be careful and avoid being shocked.
An understanding of what voltage drops to expect in conductors is required for using this approach, so if you don't understand then do not try it.
Checking voltage drop in conductors is a very valuable way of locating problems such as inadequately tightened connections. It has been a very valuable method of mine for many years. Also, it is the best way to avoid damaging the meter.