Advantages of Use of 4-port Short-Circuit Isolators

Advantages of Use of 4-port Short-Circuit Isolators

Short-circuit isolators (SCI) are intended for use in analogue-addressable loop circuits to prevent the entire fire alarm SLC loop from being disabled when a short circuit occurs in it. In this case, only the section between two adjacent isolators (closest to the short circuit location) will be affected, other loop devices will work properly. Upon removal of the short circuit condition, the isolators automatically restore power and data to the isolated section.

In case of a short circuit in a fire alarm loop without short-circuit isolators, the control panel (FACP) will not receive signals from all devices, which means the entire fire alarm loop will be disabled. This outcome does not meet the requirements of CEN/TS 54-14 concerning fault effect limitation.

One of the requirements of the standard is that a short-circuit isolator should be placed where the loop cable enters and exits a detection zone to ensure that no more than a single detection zone shall be affected by any single cable fault. The fire alarm system shall be designed to ensure that no more than 32 devices are placed between two isolators.

The requirements for the function of a short-circuit isolator are specified in EN 54-17 "Fire detection and fire alarm systems. Short-circuit isolators". A short-circuit isolator may be a physically separate (stand-alone) device or it may be integrated into another device such as a detection device, detector base, manual call point, audible or visual alarm device.

Fire alarm system manufacturers usually offer a wide range of loop devices with built-in SCI or without it – smoke and heat detectors, detector bases, input-output units, and also short-circuit isolators as stand-alone devices. Such a range provides flexibility of designing and planning of fire alarm systems for different type buildings and allows to adjust the cost of a system.

Unfortunately, most fire alarm system manufacturers in Ukraine offer only fire detectors and I/O modules with built-in isolators but don't produce SCIs as separate devices. Certainly, the use of loop devices with built-in short-circuit isolators simplifies the system design process and facilitate meeting standard requirements, but it is not effective enough for limiting multiple fault effect (i.e. caused by more than one short circuits and/or cut-offs).

Multiple faults are the most common problem in buildings with rooms and premises that belong to different owners or are for rent (e.g. shopping/business center units, residential buildings) where renovation works and other actions of people in rooms or apartments are out of official control and supervision. In buildings of this type, the probability of the fire alarm circuit damages and multiple short circuits during renovation works increases. For example, a resident can remove a detector in his or her apartment without restoring the fire alarm circuit. If actions are not taken, the system won't be able to work properly soon. Multiple faults negatively affect the functioning of the system and, therefore, people and property safety.

A fire alarm system in a building of the mentioned type can be designed as shown in the figure below (Figure 1).

Example of a fire alarm circuit with fire detectors with built-in short-circuit isolators

Figure 1. Example of a fire alarm circuit with fire detectors with built-in short-circuit isolators.

Imagine a situation where two short circuits simultaneously occurred in two apartments included in the same detection zone. In Figure 2, the part of the fire alarm loop that is isolated is in red color. In this case, fire detectors of several apartments on the floor and also detectors in the corridors are in isolated section. The situation is even worse when short circuits occur in the first and the last apartments in the loop.

Multiple fault case in a fire alarm loop using detectors with built-in isolators

Figure 2. Multiple fault case in a fire alarm loop using detectors with built-in isolators.

Specialists of our company have developed a 4-port short circuit isolator that help limit multiple fault effect in fire alarm circuits of buildings with many separate rooms (such as residential buildings, shopping or business centers, hotels, dressing rooms, etc.).
The 4-port short circuit isolator consists of two 2-port SCI in one housing. Signals are transmitted between four ports, each port (the loop ‘in’ or ‘out’ line) can independently switch from the closed state to the open state.

A part of an analogue-addressable fire detection system in which 4-port short circuit isolators are used is shown in Figure 3.

Part of a fire alarm loop with 4-port short-circuit isolators (SCI)

Figure 3. Part of a fire alarm loop with 4-port SCIs.

Figure 4 represents a multiple fault case in the fire alarm loop shown above. Only spurs of rooms where short circuits occurred are isolated, all other loop devices remain connected to the control panel.

Multiple fault case in a fire alarm loop using 4-port SCIs

Figure 4. Multiple fault case in a fire alarm loop using 4-port SCIs.

Advantages of using 4-port short circuit isolators:

  • two independent spurs of determined group of rooms/premises can be connected to one 4-port isolator;
  • the 4-port SCI provides additional reliability for a fire alarm loop by isolating a spur in which a short circuit has occurred;
  • cheaper detectors without short-circuit isolators can be used in the system or a combination of detectors without SCI in apartments (rooms) and detectors with built-in SCI in the corridor;
  • in case of a fault in the corridor loop part, only detectors between two isolators become disabled or, if detectors with built-in SCIs are mounted in the corridor, the loop transforms into two spurs;
  • usually, corridors and other common areas are accessible for anyone, so multiple fire alarm circuit damages in such zones can be repaired by a service company.

In fire detection and alarm systems of buildings with many separate rooms, 4-port short circuit isolators can be used to make separate loops for automatic fire detection function and operation of manual call points (see Figure 5). That will give an opportunity to use non fire-resistant cables for radial spurs in accordance with CEN/TS 54-14 requirements concerning fault effect limitation.

Example of use of 4-port SCIs for separation of fire alarm system functions.

Figure 5. Example of use of 4-port SCIs for separation of fire alarm system functions.

On our website you can see and download the 4-port short circuit isolator technical data.