They are normally closed wired.
Quite often a series resistor is used somewhere along the line, and sometimes very near it in the reed relay casing to avoid a tamper and bypass via an external wire ( zero omhs ) across it's terminals.
Values are around 3.3, 3.9, and 4.7 kOhms.
If say three reed switches are in series on a 4.7 kOhm circuit, best practise is if space allowes to put a 1.5 kOhm resistor in each reed switch casing if space allowes.
The loop will be sensed at 4.5 k Ohms by the alarm unit. You can can put a 200 R in the alarmbox to match exact 4.7 kOhm for the cct concerned.
edited for typo's
[This message has been edited by RODALCO (edited 03-04-2007).]
