Spend more than a cursory amount of time looking through a catalog of fire alarm cables and you’ll see that it’s often sold as shielded or non-shielded, which won’t make much sense to you if you’re not familiar with alarm circuits and a little thing known as EMI, or electromagnetic interference, which can be a big problem.
EMI: Electromagnetic Interference
Electromagnetic interference, which is sometimes also known as radio frequency interference (RFI) is a type of electrical disturbance that originates from some source outside of a circuit in question.
This type of electrical interference can interrupt or completely scramble a message being sent along an electronic line of communication, even at relatively low levels. Since the information being passed along alarm circuits can be highly sensitive, EMI is a real concern for electrical engineers responsible for constructing fire alarm systems, surveillance, and security systems such as burglar alarms.
Electromagnetic interference can sometimes manifest itself as buzzing, static, other noise; on circuits that relay video as well as audio, the same static or garbled signals could indicate the presence of electromagnetic interference.
Because it has such a pronounced effect on electrical systems that are responsible for communications and data transmission, forms of electromagnetic interference can be intentionally employed for nefarious purposes, such as signal jamming.
What Are Some Causes of EMI?
Natural sources can actually be the cause of EMI in a system. To some degree, natural sources such as the sun, the Earth’s own magnetic field, and electrical storms can all serve as the source of electromagnetic interference.
However, many manmade sources also emit EMI that can scramble the signals of radios, phones, and alarm circuits. A variety of electronic devices can release EMI that will affect other systems in their proximity.
For example, electronic devices are a big culprit of electromagnetic interference. Heavy machinery, appliances, communications systems, high voltage power lines, and even vehicle traffic can all emit EMI that has the potential to affect other electrical circuits in their vicinity. Moreover, the closer two electrical systems are to each other, the more likely they are to interfere with each other.
What Is Shielding in Fire Alarm Cables?
Luckily, there is a simple solution to blocking out electrical fields that can interfere with each other, and it comes as an interaction of the Faraday cage.
A basic Faraday cage is an enclosure made of a conductive material that interrupts the electromagnetic field around it. For example, if you’ve ever driven through a steel-trussed bridge and lost your cell phone signal, you’ve experienced the effects of a Faraday cage.
The purpose of a Faraday cage is to block out the competing electrical signals, preserving the integrity of the sensitive information being carried along the protected system.
A more basic form of this concept comes in the form of electrical shielding, which, as basic as it sounds its highly valuable. It is, effectively, a metal foil that encloses the cables on the circuit, helping to block out competing electrical fields.
Therefore, shielded fire alarm cables contain a protective foil wrapping underneath their insulation that helps to protect their communication with other nodes on the system, such as smoke detectors and alarms.
Is It Always Necessary?
The short answer is it depends. A fire alarm system, in the absence of any electrical interference, would not need shielding. The thing is, there is always some degree of interference present. Also, as a system becomes more complex and the nodes become more spaced out, the more susceptible it will be to interference. The safe answer is that all systems should be shielded, as a safeguard against risk.
If you’re looking for shielded fire alarm cables, both power limited and power-unlimited rated cables, check out EWCS Wire at EWCSWire.com. You can also contact them for more information if you still have questions, at 800-262-1598.
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