Article 19

Contactors, PMAP, and the 289H LSS

When the original 289 Loop Surveillance System was released, there was some confusion about how status contactors work. The purpose of this document is to describe how contactors can be used, how they are wired, and how to set them up in PressureMAP.

Contactor Monitoring
Contactors are useful for monitoring equipment where only two readings are needed; in PressureMAP these readings are OK and ALRM. Since contactors are used to monitor equipment that is crucial to the air pressure system, such as air dryers, it is important that loss of "continuity" in the contactor loop is monitored. In other words, the monitoring system needs to watch for the pair going "open" and the contactor not being able to provide a reading. Without these vital functions, the state of the dryer cannot be indicated.

Contactor Wiring and PressureMAP
Figure 1 shows the typical way a status contactor is wired. The basic loop consists of a pair of wires terminated by a resistor. The contact switch, with a serial resistor, goes across the pair. When the contact is "open", only the resistor at the end of the pair (the terminating resistor) is readable by either a meter or the 289H LSS. In the "closed" position, both resistors are across the pair and can be read.

The terminating resistor usually reads 540,000 (540K) ohms. When the contact switch is open, the contactor will read about 540K ohms across the pair; when it is closed, the reading will be about 270K ohms.

Figure 2 shows another way a status contactor can be wired. When the switch is open, the value of the two resistors is added together, and the contactor will read about 540K ohms. When the switch is closed, only one resistor is in the loop, and the reading will be about 270K ohms.

When the dryer (or other piece of equipment) is operating normally, the contact switch can be either open or closed. It will change to the opposite state during an alarm condition. For this reason, PressureMAP needs to know when the contactor should read OK and when it should read ALRM.

Configuring PressureMAP
Entering OPEN or CLSD in the NORM field of the editor's device screen will tell PressureMAP how to read the contactor. OPEN means the contact switch is normally open (resistance is 540K ohms) and everything is OK. The resistance will read 270K ohms when the switch is closed, and this will generate a reading of ALRM. Entering CLSD in the NORM field means that the contact switch is normally closed, and a resistance reading of 270K ohms would generate a PressureMAP reading of OK. A resistance reading of 540K ohms would generate a reading of ALRM.

How Resistance is Read
Figure 3 shows a "resistance line" indicating what this contactor will read depending on the resistance measured across the pair, based on the entry in the NORM field of the device screen. Because of changes in temperature, inaccuracies in resistors, and loop resistance (the resistance of the wires), there is a tolerance or "band" around 270K ohms and 540K ohms where the reading will remain ALRM or OK.

When the resistance is not close to either 270K ohms or 540K ohms, the device will give the error reading, PAIR, indicating pair trouble. This reading, along with ALRM, will generate a four star alarm in PressureMAP since both pair trouble and bad resistance readings create alarms. It also ensures that the dryer is always monitored.

Binary Contactors
There is another "flavor" of status contactor called the binary contactor, shown in Figure 4. Note that it is simply a contact switch across a pair of wires. When the contact is open, the circuit will read infinite resistance (actually, PressureMAP can read up to 7.6M ohms of resistance and the 289H can read up to 50M ohms). When the contact is closed, it will almost read as a short (the measured loop resistance is usually less the 1,000 ohms).

Like the contactors described earlier, the binary contact switch can be either open or closed and still be reading normally. PressureMAP decides which state the device is in by seeing if the reading is above or below 100K ohms. If it is above 100K ohms, the switch is open. If it is below 100K ohms, the switch is closed.

To tell PressureMAP which state is OK, BOPN (Binary Open) or BCLS (Binary Closed) is entered in the NORM field of the Specific Device Information Screen for the device. Entering BOPN will mean that if the contactor resistance is above 100K ohms, the device is OK. If the resistance reading is below 100K ohms, the reading is ALRM. Entering BCLS would mean just the opposite. Resistance below 100K ohms would read OK and resistance above 100K would read ALRM. Like the other contactor, an ALRM reading would generate a four star alarm.

Binary Contactor Limitations
Generally, System Studies does not recommend using binary contactors because they cannot pick up pair trouble. Figure 5 shows the resistance line for a binary contactor. If the device has BOPN in the NORM field and the pair goes open, the resistance will be 7.6M ohms and the reading will be OK, even though it should generate a four star alarm. This is not a limitation of the 289H LSS or PressureMAP, but a result of the way in which the contactor is wired.

Additional PressureMAP Contactor Readings

BUSY, VOLT, and NSE Readings
There are several other values that a status contactor can read: BUSY, VOLT and NSE. BUSY applies to a contactor that is wired onto a subscriber pair. This reading indicates that the subscriber is using the line and the 289H LSS cannot obtain a reading. This condition is considered temporary because the subscriber will eventually hang up. Therefore, it does not generate a dispatch.

VOLT indicates that there is external DC voltage applied to the pair (usually ring leakage). The voltage may originate either inside or outside the central office. This reading generates a four star dispatch.

NSE (Noise) indicates that the reading did not settle within .5 seconds, either due to excess AC voltage or some instability related to the pair such as a bad contact or connection. The AC voltage usually comes from outside the central office. The NSE reading also generates a four star dispatch.

As you can see, the status contactor is an important component of PressureMAP and the monitoring of equipment crucial to the air pressure system. We hope that this information has been helpful in clearing up some of the confusion you may have had when hooking up status contactors to a 289H LSS. If you have any further questions, please call System Studies' SupportLine at (800) 247-8255.

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