Intruder Alarm Design Considerations

designing an intruder alarm systems some basic information needs to be
gathered. Apart from the specialist regulations (BS 4737 has been withdrawn) of
BS EN 50131, PD6662, DD243 etc, BS 7671 and Part P is still relevant for cable
sizing, methods of fixing recording voltages etc. But lets start with some

1, Domestic Installations

Probably one of the most common systems. This would usually be a Grade 1 or 2x
(both audible only). A risk assessment must be carried out and this will decide
grade. Also the insurance company may have some input on a minimum grade they
will accept. Look for things like easy access points, flat roofs, weak doors
etc. These areas need special attention and may require additional protection
such as vibration sensors or door contacts. Consider how you would gain entry
yourself, this will highlight potential weak points. But generally a system
design will consist of door contacts on the front and possibly rear doors,
movement detection in the hallway, lounge, kitchen and landing. The idea of
just doing the landing upstairs is any intruder will have to cross the landing
when changing rooms. There may be a case for additional protection ie landing
window due to flat roof, master bedroom PIR
or Panic button, or high tech equipment in a study. Also garages may need some
The keypad is usually fitted in a convenient position in the hallway. Some
keypads have on-board PA features, so the keypad should always be accessible if
need in an emergency. You should also fit an internal sounder so that exit
tones can be heard throughout the exit route, and will also function as an
internal sounder. External sounders should be fitted as high as practicable,
and the cable should not be visible. You may also consider smoke or gas detection,
these are getting more popular along with ‘fobs’ for ease of use. Once you have
a basic design then you can quote the system based on the amount of equipment
As a rough guide a professional install of a grade 2x system with 4 – 5 PIR‘s
should take 2 man days. There are companies that can do it in 1 or even a few
hours but to do the install properly you should allow 2 days at your labour rate. Cables should be
routed under boards, away from any existing cabling and fixed where necessary.
Care should be taken when running a new cable through holes with existing
cables (i.e. an upgrade) as pulling the cables through too quickly will burn
the existing cables. Once you have all your cables in place from all your
points to the control location, you will be ready for the Second Fix.

Second Fix

All equipment must be securely
fixed. Normally on a plaster wall, wall plugs and screws of the correct size
for the weight of the device will be used. The only exception to this is the
sounder where you must use screws of a No. 10 in size and must protrude through
the back plate at least 2 Inches.


See the manuals for detailed wiring instructions such as tamper series loops,
or eol wiring. Most grade 1 and 2x systems will use a ‘double pole’ topology.
i.e. the alarm contacts will be wired in to 1 pair of wires and the tamer will
be on a 2nd pair. So there will be 2 ‘poles’ at the detector. EOL wiring
usually consists of 2 End Of Line resistors (value depends on manufacturer
galaxy 1k and 1k), this allows for the alarm and tamper to be on the samepair
and the panel can calculate what is in alarm by measuring the resistance of the
circuit. The higher security grade 3 systems utilise anti-mask and an additional resistor is used to monitor
the mask detector. PA devices usually only have an alarm pair.


Once your system is fully connected you need to perform various performance
tests, ie walk tests etc as detailed in the manual of the control equipment you
are fitting. Follow these steps tp check the operational performance. You
should also record the system record checks, ie the ‘readings’. These consist
of the battery charging voltage, disconnect the battery and meter between +ve
and –ve the reading
obtained should be approx 13.6 – 13.9v. AC input voltage which should be around
220-240v. The voltage at each detector, and the resistance of the cable to each
device. Alarm cable should be around 9 Ohms per 100m.


Show the customer how to use the system fully, change any default manager and
user codes. Usually you will have your own engineer code that you will program
up. While some panels allow you to ‘lock’ the engineer code to prevent
takeover, many engineers consider this bad practice so be sure you understand
the implications of you intend to use this feature. Sign the completion
paperwork and obviously collect payment.
We would advise your service contact details are clearly marked on the control

2, Commercial Installations

As with a domestic install but will probably be a higher risk, so additional protection will be required. As most commercial sites have few people to hear an alarm some sort of remote signalling is preferred, be that a voice/speech dialler or monitoring via an alarm receiving centre.

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