INITIATING CIRCUIT FAULTS

Initiating circuits, like power supply circuits, may experience shorts, opens, or ground faults. Operating tests of initiating circuits to locate and repair faults are best performed after normal working hours to avoid disruption of normal activities.

Short Circuit

A short circuit between two points on the same side of the circuit does not harm system operation and is normally not detected unless the short circuit also involves one or more ground faults. A short circuit between wires on opposite sides of the circuit causes an alarm. A clue to this

condition is the fact that an alarm condition exists for an initiating circuit, but inspection shows that none of the initiating devices connected to that circuit have operated. The following troubleshooting steps will guide you in finding and repairing the fault:

1. Tag and disconnect the initiating circuit loop at the control unit or annunciator terminals.

2. Measure the initiating circuit resistance with an ohmmeter. A value of 100 ohms or less confirms a short circuit. The lower the value, the closer to the source the fault is located. A measurement equal to the end-of-line resistor value or slightly higher is normal and suggests looking in the control unit or annunciator for the fault. (Determine the proper value of the resistor, usually 1,000 to 2,000 ohms, from reference materials on the equipment.)

3. If resistance measured is low, confirming a short circuit, move to a point that is electrically about halfway between the source and the endof-line resistor for the next resistance measurement. A low resistance, near zero, indicates the short circuit is quite near the test point. A resistance of 50 to 100 ohms indicates that the circuit is a long one of smaller gauge wire and that the short-circuit fault is near the end of the circuit. At each new test location, break both sides of the circuit by disconnecting wires at a convenient initiating device or junction box. Measure circuit resistance in the direction toward the end of the circuit.

4. Low resistance measured from the second location, less than 100 ohms, indicates the short circuit is still farther toward the end of the circuit. High resistance, approximating the end-of-line resistor value, indicates that the short circuit is closer to the control unit.

5. Continue moving toward the short circuit, dividing the circuit approximately in halves each time, and repeat the measurement of resistance toward the end of the circuit using the guidelines in Step 4 as the rule for interpreting each succeeding measurement.

6. When the fault is located, repair it, reconnect the disconnected wires, and restore the circuit to normal service.

Open Circuit

An open-circuit fault in an initiating circuit stops the supervising current. The trouble relay at the control unit or annunciator deenergizes, and trouble indicators are activated

for the circuit. Initiating devices closer to the control unit or annunciator than the open fault may continue to function. Devices beyond the fault cannot operate. If an open-circuit fault occurs, turn off any audible trouble signals by operating the trouble silence switch. Continue troubleshooting by using the following steps:

1. Tag and disconnect the initiating circuit loop at the control unit or annunciator terminals.

2. Measure the initiating circuit resistance with an ohmmeter. An infinite reading (no change in meter reading from the reading with the meter disconnected from the circuit) confirms an opencircuit fault. A measurement equal to the end-ofline resistor value, or slightly higher, is normal and suggests looking in the control unit or annunciator for the fault.

3. If the open-circuit fault is confirmed, leave the two circuit wires off their terminals, taped separately. Move to a point that is electrically about halfway between the source and the endof-line resistor for the next resistance measurement. Choose a convenient initiating device or junction box and measure resistance across the two sides of the initiating circuit. If the measurement is still infinite, the open-circuit fault is still farther along the circuit toward the end-of-line resistor. If the measurement is now about equal to the end-of-line resistor value, the open-circuit fault is between the present measurement point and the source.

4. Move toward the fault to a point electrically about halfway between the present measurement point and the end-of-line resistor or the source, and measure resistance across the circuit.

5. Infinite readings indicate the open-circuit fault is toward the end-of-line resistor from the new test point. Readings approximating the endof-line resistor value indicate the open-circuit fault is toward the source.

6. Continue taking new readings, advancing toward the fault. Look especially for loose connections at splices and at initiating-device screw terminals.

7. When the fault is located, repair it, reconnect wires at the control unit or annunciator, and restore alarm service.