Definition
Line seizure
Also known as: line grab, line seizure module, RJ-31X seizure
Line seizure is the ability of an alarm panel to disconnect any other device sharing its telephone line and take exclusive control, so it can transmit an alarm even if someone is mid-call.
How it works
The panel is wired ahead of the premises telephones, conventionally through an RJ-31X jack. When the panel needs to report, it opens the connection to the downstream phones and seizes the line, guaranteeing a clear path to the monitoring center regardless of what else was happening.
Why it matters
Without seizure, an off-hook telephone — or an intruder deliberately taking a phone off-hook — can prevent an alarm from being reported. Any replacement that shares a line among devices has to preserve seizure behavior, or a documented safety function is silently lost.
Common use cases
- Fire alarm panels sharing a premises line with telephones
- Burglar alarm panels guaranteeing a reporting path during an intrusion
Giving each panel its own FXS port sidesteps the problem entirely, which is usually simpler than reproducing seizure behavior on a shared port.
Common misconceptions
-
“Line seizure is unnecessary if the alarm panel has its own dedicated line.”
On a genuinely dedicated line there is nothing to seize, which is fine. The requirement matters wherever a line is shared — and shared arrangements are common in smaller buildings.
Frequently asked questions
It has to be designed for it. Giving the panel its own FXS port is the cleanest approach, since the port is then dedicated and seizure is moot. Where sharing is unavoidable, the appliance must implement seizure explicitly.
The jack conventionally used to wire an alarm panel ahead of premises telephones so the panel can seize the line.
Related to this term
Related terms
Also referenced
Sources
- NFPA 72: National Fire Alarm and Signaling Code (opens in a new tab) — National Fire Protection Association
Last reviewed .
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