Relays, Interference and Codes, Oh My!
This month, I am focusing on two topics of great importance to fire systems installers and technicians: end-of-line relays and electrical interference. For those new to the field, I have included a special sidebar about dealing with fire and building codes.
How to Use End-of-Line Relays
When a fire technician installs a 4-wire smoke detector, an end-of-line relay (EOLR) must be installed at the end of each 4-wire initiating circuit. An EOLR is used to maintain supervision of a smoke detector operating power up to, and beyond, the last detector.
Unfortunately, novice fire technicians often do not understand the importance of the EOLR.
An EOLR is a device designed to supervise the DC power that feeds one or more 4-wire smoke detectors. Usually, from an outward appearance standpoint, an EOLR looks like a small box with five or six wires protruding from it.
Power monitoring takes place through a set of dry, normally closed (NC) contacts inside the EOLR module. These contacts are magnetically linked to a coil (see drawing). This coil is in turn powered by the same 12VDC or 24VDC that operates the smoke detector(s).
Here is how it works. As long as power is provided to the last 4-wire smoke detector, the coil within the EOLR will remain energized. This will keep the NC contacts in their closed position, thus maintaining the integrity of the circuit. As long as this state of affairs continues, the fire alarm panel will continue to “see” the end-of-line (EOL) resistor.
Minimizing Electrical InterferenceElectrical interference through high-voltage inductance can be a killer in any low-voltage system. This is why some experts claim that the best way to avoid inductive crossover is to maintain a distance of 10 inches for every 100V between low- and high-voltage wires. This is especially true of cables that run parallel to one another for relatively long distances.
Just like audio and video, fire alarm initiating circuits are vulnerable to high-voltage electrical interference. Most of the time, this interference occurs because of nearby high-voltage wires that induce an AC current onto the conductors within a fire alarm cable.
For example, an electrician installed a conventional fire alarm system in a large commercial building. It did not take long until a problem surfaced. According to the electrician, every time the overcurrent protection on a large fan motor in a ventilation system abruptly disconnected a large fan motor, the initiating circuit for a smoke detector in the equipment room initiated a trouble signal in the fire alarm panel.
The fire alarm trouble signal was how the owner of the building knew that the fan in the ventilation system kicked off. The trouble signal on the smoke zone was the only means by which employees of the client knew that the fan motor had to be reset.
In this case, the electrician made the mistake of installing the fire alarm cable for the utility room smoke detector in the same raceway that contained the high-voltage, three-phase electrical wires that powered the ventilation system.
Inductive interference occurred in this case because the overcurrent protector abruptly stopped the current flowing to the ventilation fan motor. When this current suddenly stopped, a back electromotive force, or a back EMF, took place in the motor circuit.
Because the fire alarm wire was in close proximity to the fan circuit, an AC surge was introduced into the fire alarm initiating device circuit. This caused the fire alarm control panel to go into trouble mode.
The preferred method is to reroute the fire alarm cable that carries the initiating device circuit so it is isolated from the high-voltage power cable in a conduit or raceway. Of course, it will cost an alarm company in terms of labor because it will take time to enact. However, this method will bring almost immediate results.
To provide you with a follow-up on the electrician with a peck of trouble, he ended up rerouting the fire alarm cable. He did this by removing it from the raceway that housed the ventilation electrical conductors. The outcome was “no more false trouble conditions.”
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