You're asking a very astute question about elevator recall systems. It gets to the heart of how these relays are designed to work cooperatively. Here's a breakdown of how the system coordinates the activation of the Primary and Alternate Recall Relays using a single Signaling Line Circuit (SLC)

Understanding the Goal

The primary objective is to ensure that:1. If a fire alarm condition is detected, the elevator is recalled to a safe designated landing (typically the lobby).
2. One relay and only one should activate based on the specific conditions. Typically, the Primary relay handles the initial recall. The Alternate relay is a backup in case the Primary relay fails or the primary recall landing is compromised.3. The system needs to be supervised to detect faults in the wiring or the relays themselves.

How the SLC Wiring Signals the Relays

The SLC, along with the end-of-line resistor, provides the Fire Alarm Control Panel (FACP) with a monitored circuit. The FACP constantly polls the SLC to check for changes in its electrical state (voltage or current).The key here is that the relays themselves are designed with different "sensitivity" or responsiveness to the SLC current or voltage, and often incorporating timing elements.

General Principles and Methods

While the specific implementation varies between manufacturers, here are the most common methods used to selectively activate the Primary or Alternate relay:1. Voltage/Current Thresholds

The FACP changes the voltage or current on the SLC to signal a recall condition. The Primary and Alternate relays are designed to respond to different voltage or current thresholds.Example: The primary relay might activate when the SLC current drops to 20mA and the alternate only activates when it drops to 10mA2.

Timing

The primary relay begins counting after the SLC condition changes. If it clears before a specific time, it will not activate The alternate recall relay begins counting after the SLC condition changes. If the primary does not activate, it will activate after the set time.3.

Addressability and Protocol (More Advanced)

In more modern systems, the SLC is an addressable loop. Each relay (or interface module connected to the relay) has a unique address. The FACP sends a specific command to the address of either the Primary or Alternate relay. The relays are programmed to only activate when they receive a command with their specific address. This is the most flexible and reliable method, but it requires more sophisticated hardware and communication protocols.

Example Scenario (using Voltage/Current Thresholds)


Normal Condition

The FACP sends a normal current signal (e.g., 25 mA) on the SLC. Neither relay is activated because the current is above both of their threshold values.2.

Fire Alarm

The FACP drops the SLC current to 20 mA. The Primary Relay is designed to activate at or below 20 mA. It trips, initiating the recall sequence. The Alternate Relay is designed to activate only at or below 10 mA. Since the current is at 20 mA, it remains inactive.3.

If Primary Relay Fails

If the Primary Relay malfunctions and doesn't activate (and the FACP is programmed to detect this, perhaps through a feedback signal), the FACP might then drop the SLC current further to 10 mA. Now, the Alternate Relay activates, taking over the recall function.

Supervision and Fault Detection

The end-of-line resistor (EOLR) plays a critical role in supervision. The FACP constantly monitors the resistance of the SLC. Any of the following problems can be detected: Open Circuit

If the wire is cut or disconnected, the resistance goes to infinity (open circuit).

Short Circuit

If the wire shorts to ground or another wire, the resistance drops to near zero.

Relay Failure

Some systems have monitoring contacts in the relays. If a relay fails to activate when commanded, the FACP receives a fault signal.

Important Considerations



Manufacturer-Specific Design

The exact method used to select the Primary or Alternate relay is proprietary and varies between manufacturers. Refer to the manufacturer's documentation for the specific system you are working with.

Proper Configuration

It is crucial to configure the FACP and the relays correctly according to the manufacturer's instructions. Incorrect configuration can lead to system malfunctions and safety hazards.

Testing

Regular testing of the elevator recall system is essential to verify that both the Primary and Alternate relays are functioning correctly.

In summary: The clever use of voltage/current thresholds, timing delays, or addressable protocols, combined with continuous supervision, allows a single SLC to selectively activate the Primary or Alternate elevator recall relays, ensuring a robust and safe response to fire alarm conditions. Always consult the specific manufacturer's documentation for the particular system you are working with. Flag for review