Showing that the maximum value of capacitance for the Signaling Line Circuit (SLC), the installation manual specifies that capacitance can be no more than 500 nano-farads (nF), which is also referred to as 0.5 micro-farads (mFd).
The maximum capacitance value was designed into the fire alarm system by the engineers at the manufacturer. The engineers have made sure the system will work as long as the capacitance of the wires on the SLC is lower than 500 nF. Higher capacitance will distort the data being sent on the SLC too much, and the whole system will shut down.
In the process of listing for use, a testing laboratory, like UL, ULC, CE, CCC, etc. has tested this value of capacitance for the SLC and found that the circuit works, as long as the capacitance is below 500 nF.
Once the testing laboratory has actually tested the entire fire alarm system, including the maximum capacitance value for the SLC, the testing laboratory places the entire fire alarm system on their list of systems that work. (At least as long as the system is installed according to the listed specifications in the maintenance manual.)
The testing laboratory has tested capacitance of the whole SLC wiring circuit, without devices, modules, panels, or any other electrical or electronic components. The value of capacitance in the listed specifications, showing 500 nF, does not show the frequency that this capacitance is to be measured at.
The capacitance of the wires in the SLC can be measured by using a capacitance tester, available on the web.
The capacitance test involves testing the installed circuit, including all the old wiring and the newly installed wiring, in a single measurement. None of the devices can be attached to the completed circuit wiring, of the test will be inaccurate.
The capacitance value shown in the maintenance manual is the reference that the fire alarm installer has to match.
Capacitive Reactance Test
The manufacturer and the testing laboratory have not tested the capacitive reactance of the wires for the SLC.
Because the capacitive reactance test is not included in their listing, there is no reference to this in the listed installation manual. Without this reference, any value obtained from the test can't show whether the circuit works or not. It can't show if the circuit will be acceptable, or not acceptable.
Yes, the capacitive reactance test can be performed; no, the test won't show whether the SLC circuit works.
Real Life Problems with Old Wiring
Some of the other problems with using old wiring for a fire alarm system are caused by induced signals. Once in a while, when these problems exist, the issues show up as constant or intermittent "ground faults", low level background noise in fire alarm speaker circuits, or constant or intermittent troubles in communication.
These symptoms are not addressed in the installation manual for the fire alarm system, but these still have been a problem. Many of them I've had to fix myself because the fire alarm manufacturer doesn't know what is causing the problems.
Some of the induced problems can be found and fixed before changing over the fire alarm system, some of these induced problems have to be found and fixed once the new system is installed and is fully operating, and a very few of these problems take years before they are noticed.
Just be careful when using old wiring; surprises happen, and fixing the surprises is never fun.