Testing for Normally Open Circuits

Some circuits, in home security systems, are normally open. This means that switches are closed when a door or window opens or vibration (such as breaking glass) occurs. A typical circuit is shown in Figure 2-5. To test a circuit such as this, set the switches to NORMALLY OPEN and MOMENTARY and then turn POWER ON. Now connect one test lead to one end of a wire to be tested and the other test lead to the other end of the wire to be tested. The buzzer will not sound if the circuit is open. Closing any switch will cause the buzzer to sound.
Testing for Normally Closed Circuits
Also, some circuits, such as home security systems, are normally closed. This
means that switches are opened when a door or window opens or vibration.
occurs. A typical circuit is shown in Figure 2-6. Totesta circuit such as this, setthe switches to NORMALLY CLOSED and MOMENTARY and then turn POWER ON. Now connect one test lead to one end of a wire to be tested and the other test leaa to the other end of the wi re to be tested. The buzzer wi II not sound if the circuit is closed. Opening any switch will cause the buzzer to sound.

Testing for Intermittent Closures

Sometimes wires or circuits wi [I be intermittently open. For example, jiggling or bending a wire may cause the wire to break open for a short time and then close again. The same thing may happen in reverse. When two wires are close togethe: or switch contacts are very close, there may be no continuity until the wires are bent or stretched, at which time the wires or contacts may touch. There may ever be cases where wires have continuity (or no continuity) until the temperature rises.
_ :: :ce-:::.~ level, at which time the wire opens or two wires touch as the ~ :::_::~ or contracts. A typical case is illustrated in . _ :ce’, ittent openings or closings are hard to find. A latching continuity l8lle -::. -,0′ i-em easier to find by detecting very short openings or closings .
. ~ _ ~ _: :::e so short that you would not hear the buzzer - a hundredth of 1lE:J:iTC : - eo~. The latching continuity tester detects these small openings or
:: _::: :-en sounds the buzzer continuously until the tester is reset. You can ~ :_::: _ –e continuity tester to a circuit overnight. If you return in the morning _._~ : _::zer is on, an intermittent closing or opening occurred sometime
.~ ~-:
“: :::e:ect intermittent closings, set the switches to NORMALLY OPEN and
_’” _ = _ ::: :::,nnect the test leads to the circuit. Press the reset button if the buzzer is .,;;:”-,_’: _~. The buzzer should now be silent but will sound if the circuit is closed _ :::’ier period of time, or is permanently closed.
“: ::etect intermittent openings, set the switches to NORMALLY CLOSED “,e _:. -CH. Connect the test leads to the circuit.