Able with cable

All of the different cabling forms used by installers have been subject to major changes over the years in order to embrace better forms of communications and remote signalling.

In keeping with such advances the actual techniques of wiring even the most basic detection devices and components have also developed. An ever-increasing level of monitoring by security system control equipment is now offered.

This is reflected in the Level 3 Certificate of Knowledge of Security and Emergency Alarm Systems 1852 January 2005 scheme handbook which includes objectives within the syllabus on the principles and uses of line monitoring devices.

In the intruder division the EN Standards now require interconnections to be monitored in relation to the system grade. Unfortunately, because there is no mandatory format and no specific restrictions on interconnections, manufacturers use a number of configurations.

This means the installer must appreciate the logic of them all.

So how is monitoring actually applied in practical terms by the installer throughout our industry?

Monitoring basics

In traditional, basic contact switching and line monitoring schemes, diodes would be placed in parallel across the switching devices with a terminating resistor in series.

With the system 'unset' the diode would only allow the passage of current through the switching devices in one direction and randomly short them out as they open.

However in 'set' the polarity could be reversed so that the diodes would inhibit the passage of current and the contact opening could then drop out a continuously rated relay. This meant a conventional circuit could always be permanently energised and monitor detection devices and components when configured normally closed.

Release modules could be used to respond to removed voltages and then generate outputs to achieve simple monitoring.

This 'original generation' relay-operated control equipment has now been replaced by solid state integrated digital logic panels so such monitoring functions are now performed electronically.

In the intruder sector, detection loops have traditionally been wired double pole with the two circuit conductors at different polarities so both alarm and tamper protection is provided.

New generation panels

These 'new generation' panels have loop resistance limits in the thousands or tens of thousands of ohms and loop currents in the micro amp range. By using such set-ups, although we could say that the cables wired by this method were supervised, they are still only fully supervised when used with line monitoring devices.

Using line monitoring, the circuits are normally closed but the current flowing in the circuit is constantly measured and analysed to detect a given impedance within the circuit.

Any change in current magnitude or direction will create a response at the control equipment. In its simplest form, a single end of line resistor (EOL) would be used for the detection loop. These closed control loop systems have a much lower limit and operate rather differently.

For most resistors, if the temperature is constant, the potential difference across it is proportional to the current through it, since we know that the electrical resistance of the EOL impedes the flow of electrical current.

The EOL device is placed at the 'far' end of the loop and in series to pass a limited current. Therefore opening any wire of a closed circuit device, either by the detection device opening electrically or if a wire was to be disconnected, drops the current to zero so signalling an alarm.

Shorting the wires bypasses the EOL causing the loop current to increase, which also signals an alarm.

With the trend towards higher grades of monitoring and supervising of cables we will see an even greater drift apart between the major equipment formats ...

Easily installed

These systems are easily installed by those installers that are more familiar with traditional double pole wiring because the resistor legs can always be fitted either way round. They are not polarity dependant.

As an extension on EOL wiring we also find a range of dual operation techniques. These use a variety of series and parallel resistors with the latter devices fitted in parallel across the switching contacts. Unfortunately there is a proliferation of systems, resistive networks and component values used by the competing manufacturers to achieve this dual operation and a reduction in cabling. This means retrofitting control equipment is never really an easy option without amending the loop wiring.

To illustrate this wiring we can refer to Fig 1. This shows typical dual operation or full supervision with the detection devices wired as normal and a 4k7 resistor fitted in parallel with the complete loop. Fig 2 illustrates the most popular variant forms of monitoring detection devices and the advantages and disadvantages of each.

The future of cable monitoring

Even if we think back to old style fire detection systems, it was never an easy exercise to retrofit different control equipment within existing installed conventional cabling systems.

This was due to the different monitoring devices used by manufacturers in the detection devices, call points and signalling equipment.

Yet, in theory, the principles of monitoring and installing the components were exactly the same. This has now become further complicated by the advent of addressable systems and combination wiring which enables detectors and sounders to be installed together on zone detector lines. Separate sounder lines are also used and have particular monitoring installed.

As technology progresses the need for integrity of all cabling, connected devices and signalling equipment is becoming increasingly complex. This applies now to every system in which installers have an interest.

Although all of the detection devices and signalling components used in the intruder sector and most locks and ancillary parts used in access systems may be compatible, the actual monitoring formats used by the control equipment may differ enormously. With CCTV networks the cable monitoring and detachment of connected cameras in professional systems is always shown as a prompt in the form of a video signal loss at the monitor.

What is certain is that monitoring techniques are something that the younger installer should try to understand or costly errors can be made, especially if no budget has been made for wiring modifications. Even control equipment claiming to be the same BS or EN cannot be substituted without involving the wiring and monitoring devices of all the connected system components.

This applies to an even greater extent as we become more involved with complex systems and higher security grades.

Installers have always accepted that the control circuits used to collect data by addressable and polling techniques and configurations to link the more complex parts of equipment could never interchange with other systems.

As we improve our security monitoring this philosophy is becoming more enshrined in even standard detection and signalling methods.

The end result

It is fair to say that in all security systems we can always add simple components because they remain compatible with all of the main equipment formats. However problems occur when we wish to change the main controlling equipment. This is because the values and responses of the monitoring components are not necessarily similar.

I suspect that with the trend towards even higher grades of monitoring and supervising of cables, we will see an even greater drift apart between the major equipment formats.

At the risk of being rather dramatic, it means that security installers will have to remain even more high tech in their working practices than we would ever have imagined all those years ago!