Douglas Krantz - Technical Writer - Describing How It Works

What does SLC Loop Shorted Mean?

By Douglas Krantz | Maintenance

What does SLC Loop Shorted Mean?

What does SLC Loop Shorted Mean?

Greetings Douglas,

I've got a question about a fire alarm panel. It has an "SLC LOOP SHORTED" message on the display, and it has no power on the SLC terminals.

All of the smokes don't have any blinking lights. I pulled every one out and check to see if panel even noticed, and it doesn't. Any ideas?

Thank You, AB

We'll start with the SLC (Signaling Line Circuit).

The SLC is the circuit that carries the signals between the devices in the rest of the building, like the addressable detectors and modules, and the panel. It also carries power from the SLC power supply in the panel to the addressable devices in the building.

Electrical Short

The word "Circuit" implies "Circle". For the electrons, the pathway of the circle is:
  • Out of one terminal of the SLC power supply at the panel
  • Through one of the wires of the SLC circuit
  • Through the devices
  • Through the other wire of the SLC circuit
  • Back into the other terminal of the SLC power supply at the panel
  • Back through the SLC power supply, itself

By following this pathway, the electrons can be used over and over again through the SLC circuit.

The word "Short" means that the electrons are not taking the regular path through the addressable devices, but taking a "shortened" electrical path.

Weak SLC Power Supply

The SLC power supply is a very weak power supply. If the resistance of the SLC wiring is reduced, so it draws too much current from the power supply, the excess current will draw down the voltage of the SLC power supply.

In other words, the current through the shortened electrical path is pulling the voltage of the SLC power supply to zero.

The SLC power supply normally provides power to the blinking lights on the devices; being zero volts, the SLC power supply isn't providing power to the blinking lights. The lights can't blink if they don't get power from the panel.

There are many causes of a "Short Fault". The electrons in the building's SLC circuit are taking a shortened path:
  • Through a wire-to-wire short on the SLC
  • Through a bad component inside an addressable on the SLC
  • Through a reverse-wired device on the SLC
  • Through water on the wires of the SLC
  • Through a component inside the panel (doesn't happen very often)

Any one of these can be the "Short". The panel isn't smart enough to know the difference, so the panel can't say anything other than "SLC LOOP SHORTED".

Pulling the SLC Voltage to Zero

Somewhere in the building, there is a short on the SLC circuit.

Your ohmmeter will detect a wire-to-wire short. However, it won't detect a bad component in a device, a reverse wired device, or many of the water-on-the-wires faults.

When looking for any short on the SLC, instead of using the ohmmeter, look for the presence of voltage to show when the circuit is not shorted. If the SLC is shorted, the SLC voltage is zero; if the SLC is not shorted, the SLC voltage is present.

If there had been a short, some panels will take up to a minute before checking to see if the short is gone before providing voltage again, so be patient.

SLC Voltage is not Steady

The signals on the Signaling Line Circuit (SLC) prevent your voltmeter from giving you a steady voltage reading; the voltage will always jump around on a working SLC. Get used to it.

What you look for is a normal "Unsteady Voltage is on the Circuit", or a faulty "No Voltage is in the Circuit".

Douglas Krantz
Life Safety
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