The Devil's in the Details

For the last century, electricians have supplied the superhighway over which electrical apparatuses in homes, retail stores, commercial and industrial facilities, and government institutions operate. No matter where or what it is, if it requires 120 AC, 240 AC, 440 AC or more, an electrician must provide the infrastructure.

Electrical contractors (ECs) also have assumed the role of low-voltage installers, installing the highway over which low-voltage data and applications flow.

In most cases, this involves the use of a specialized low-voltage firm. But as more electrical contractors assume the role of low-voltage specialist, essentially providing turnkey solutions, the job of low-voltage installation has taken on an entirely new life.

However, there are unforeseen problems when the EC provides turnkey low-voltage system installations, and these can kill a job, such as when technicians fail to effectively deal with the problems.

Under normal conditions—where an outside low-voltage contractor is used—this rarely impacts the EC. But when the EC elects to act as his own low-voltage contractor, it becomes his job to provide the expertise necessary to successfully produce a turnkey system.

Take a keypad installation, for example. Without a low-voltage technician on the job, it’s the EC’s job to know the wire gauge, whether it needs a shield and if twisted pairs are needed. Using the wrong cable can mean that data won’t successfully travel from one end of the cable to the other.

In the area of fire alarm systems, ECs commonly work with national firms, such as ADT and Simplex. Because so many of the fire alarm systems they install use shielded, power-limited fire alarm (PLFA) cable, ECs commonly assume that shielded cable is used on all fire alarm jobs. That is not necessarily true.

Past experience and conventional wisdom are not always an EC’s best guide. For example, in an addressable, data-driven system, the manufacturer may require simple nonshielded cable that uses a global twist. Others require the use of shielded wire, all of which is determined by the manufacturer’s technology.

There also is the conductor-size issue. Conventional wisdom says that for long runs, 16 AWG is better than 20 AWG, and sometimes it is. Yet, when it comes to modern day addressable, data-driven systems, manufacturers often specify conductor sizes of 20 to 18 AWG maximum.

The issue of whether to use a shield, as well as that of wire gauge, involves the capacitive and inductive properties of the conductors inside a cable. Often, the use of shielded cable comes at a cost of distance. Many ECs have to repull the wire on a job because they made assumptions without checking the manufacturer’s installation instructions.

The EC needs to know the equipment. When the EC works with a low-voltage contractor, the cable is the only thing he usually has to consider upfront. This specification often is given to the EC before the job starts.

In access control, ECs who install Wiegand-type proximity readers commonly come to know that six-conductor cable is required between the reader and central processing unit (CPU). But this is not always the case.

It is necessary to install seven wires, for example, when connecting an AWID Sentinel-Prox, model MR-1824, with a Brivo ACS-3100 access control system. Or when installing a Star RFK101 proximity/keypad unit with the same Brivo system, seven conductors are needed. There are some reader/keypad combinations that require up to 10 conductors, so it is absolutely necessary to know the specifications of the equipment used beforehand.

Another example where foreknowledge is required involves passive infrared (PIR) detectors used in burglar alarm systems. No matter where the electrical engineer or architect places one on the blueprint, it’s the EC’s job to know what works and what doesn’t.

For example, conventional PIRs require no more than four conductors to make them function (two for power, two for detection). But when a PIR is addressable, that equation can change. In the case of a DSC model AMB300, only two wires are needed (two for power and detection).

Placement of PIRs also is an important issue, and the EC must know how these devices work in order to do an effective job.

Take, for example, a job where the specification calls for two PIRs directly facing a parking lot through two plate glass windows. The problem is false alarms can occur if a car pulls into the lot at night and its headlamps shine directly into one of these detectors. Although this is not a common problem, it is one that can occur. The best solution in this case is to relocate them to areas of the room where white light cannot reach them.

It’s important that ECs acting as the low-voltage contractor know and understand how these devices work in order to ensure trouble-free operation.

COLOMBO is a 32-year veteran in the security and life-safety markets. He currently is director with and a nationally recognized trade journalist located in East Canton, Ohio.


About the Author

Allan B. Colombo

Freelance Writer
Allan Colombo is a 35-year veteran in the security and life safety markets. He is director with and a nationally recognized trade journalist in East Canton, Ohio. Reach him at

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