Douglas Krantz - Technical Writer - Describing How It Works

How is the Capacitance of the Signaling Line Circuit (SLC) Measured?

By Douglas Krantz | Life-Safety

How is the Capacitance of the Signaling Line Circuit (SLC) Measured?

How is the Capacitance of the Signaling Line Circuit (SLC) Measured?

Greetings Douglas,

I have a fire marshal that is asking me to demonstrate that a buildings SLC wiring is within a tolerance of manufacturers specs of .5 mF before we install our new equipment.

He wants us to simulate the amplitude and frequency of the system and show him.

I am at a loss as to how I could do this and any help would be appreciated greatly.

Thank You, JM

Keep in mind that your fire marshal has a genuine concern that the Signaling Line Circuit (SLC) cable's capacitance is not going to corrupt the data. The manufacturer says the cable's cumulative total capacitance for the circuit cannot exceed 0.5 microfarad (mfd). Usually, the specifications from the manufacturer don't go into why the capacitance cannot exceed 0.5 mfd, the specifications only say that 0.5 mfd is the limit.

The problem that you have to deal with is that the measurement is determining the capacitance of the cable, not the devices. Just adding the devices messes up the measurements, making the results meaningless. Yes. You have to remove all the devices, and then splice together wire-ends inside the back-boxes so the whole Signaling Line Circuit (SLC) is still connected together. Then make the measurements.

To check out the installed capacitance of the cable, you can use an inexpensive capacitance checker found on many multimeters commonly carried by fire alarm technicians. When making the measurements, just make sure that all wires are spliced through, and there are no devices, panels, or even resistors connected to the wires.

Your real job, however, is going to be educating the fire marshal of the technique you are using. In essence, even though the capacitor checker on the multimeter is using a signal at some frequency, the frequency of the signal it uses has nothing to do with the outcome of the measurements.

Baud Rate, Not Frequency

When talking to technical support, they can tell you the baud rate of the data, but they can't tell you the frequency of the signal. Even the fire alarm system designers at the manufacturer really can't ever answer the question of what actual frequency is being sent through the wires. That's because the data isn't a continuous sinewave, measured in frequency like a radio or TV transmitted signal; the data is squared-off signals, similar to square waves, measured in baud, and sent at random times.

When a signal is being sent at, say, 9600 baud, the range of frequencies needed to send the 9600 baud can easily be from 4,800 Hz (4 kHz) all the way to 100,000 Hz (100 kHz). Because of the random nature of the data being sent, a spectrum analyzer might show that the frequencies go lower than 2 kHz. The amplitude of these signals is high at the low-frequency end of the spectrum, and falls off to nearly zero toward the high-frequency end.


It's the capacitance that needs to be measured.

Capacitance in a cable or in a component, on the other hand, doesn't change with frequency; at any frequency, capacitance is the ability to hold electrical charges. Like a bucket holding water, the size of the bucket doesn't change, no matter fast the water is poured in or dumped out.

The amount of capacitance in a cable, however, is extremely important; too much capacitance, and the data signal becomes corrupted. Think of the bucket again. If, at a certain rate of filling and dumping the water out, every time the bucket is only half full when the water is dumped out, like a capacitor that is never fully charged, the bucket is never going to be full.

The capacity of the bucket has to be small enough for the water to fill the bucket, and the capacitance of the cable has to be low enough that the electricity fills (fully charges) the capacitor.

Accumulated Wire Length, Not Distance

Capacitance of a cable is directly affected by the quantity of cable making up the circuit. The more cable in a circuit, the greater the capacitance. Using a total of 50 cables that are each 20 feet long, and all home run to the control panel, is exactly the same as 1,000 feet of cable in a single run; both circuits use 1,000 feet of cable.

The type of cable is also important.
  • Larger diameter wires in a cable have more capacitance

  • Thinner insulation around each of the wires increase capacitance

  • The type of insulation around the wires affects the capacitance

  • Shielding the cable greatly increases capacitance

The fire marshal knows that the requirements show what should be done, but that the actual installation sometimes don't quite meet the requirements. What the fire marshal wants to do is confirm that the installed wire meets the requirements of the maximum capacitance.

Douglas Krantz
Life Safety
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