Burglar alarm phone wiring

Why does my broadband drop out?

Burglar alarm systems and ADSL broadband don't play well together!  But don't despair!  With correct wiring, there's no reason when the two can't co-exist happily.  Often when people get broadband installed they already have an alarm wired into their phone line.  In such a case, the self install kit broadband option is probably not the best choice.  To minimise disruption, a Central ADSL Filter needs to be hard-wired in to the system by a technician.

Central station reporting

Burglar alarm systems typically report events to central monitoring stations by placing phone calls using you normal phone line.  (Note than some modern systems have options for internet TCP/IP based reporting.)  However, there is a potential problem with reporting events over the phone.  Normally alarms have an entry delay to give someone sufficient time to disarm the alarm before it goes off.  The problem is, this gives a burglar enough time find the phone, pick it up, dial some random number, and leave it off the hook.  Now when the alarm tries to place a phone call (after the entry delay), it finds the phone already in use and can not report the break-in.

To overcome this problem alarms have a line interruption feature.  Before dialling to report an event, the alarm interrupts the phone line, effectively disconnecting all the phones in the house, and freeing the phone line for the alarm to place its call uninterrupted and report the break-in.  For this feature to work, the alarm must be wired correctly.  The phone line must first come into the main alarm panel before being distributed to the rest of the house, as shown below.

ADSL broadband internet

ADSL is a common method of delivering high-speed internet over the same cable used for the normal phone line.  The same cable can be used because the ADSL signals use a different frequency band to normal phone calls.  For both systems to co-exist happily, filters must be used to separate out the two signals, otherwise when a phone is picked-up, it will interfere with the ADSL signal, causing the internet connection to go down.  Typically people who have ADSL broadband internet choose the self install option to save money.  This involves placing filters on every phone in the house as shown below, so when they are used they do not interfere with the internet.


Figure 1 - Incorrect ADSL and alarm phone line wiring.

Alarm interfering with internet

Of course, with this standard set-up, when the alarm reports an event it not only disconnects the the phones, but it also disconnects the ADSL internet.  This can be avoided with a different wiring set-up, shown below.  Instead of putting individual filters on every phone, one central filter can be hard-wired into the phone line as it enters the house.  This separates the ADSL internet signal from the phone signal at the point where the phone line comes into the house, and before the main alarm panel.  In this set-up, a separate broadband wall socket is provided to plug in the ADSL modem.  Because the internet connection does not pass through the main alarm panel, when the alarm dials-out to report an event it does not interfere with the broadband connection.  Note that this type of wiring needs to be performed by a qualified technician.


Figure 2 - Correct ADSL and alarm phone line wiring.