---
title: How to tell if two sensors activate at the same time - Cross Sensor Zones
description: How to tell if two sensors activate at the same time - Cross Sensor Zones
---
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# How to tell if two sensors activate at the same time - Cross Sensor Zones
Cross-sensor zones are a way to set two separate sensors to trigger an alarm when only both have been triggered. This is used to tell if two sensors activate at the same time.
For example:
Cross Sensor Zones 99-100 will create a cross-reference between the zones to trigger an alarm only when they both activate
and will cause trouble condition if only 1 activates while the other doesn't
#### Here is a quick guide:
Zone 99 triggers and Zone 100 Does not: **Zone 99 will receive a trouble condition, and no alarm is sent.**
Zone 99 does not trigger but Zone 100 Does: **Zone 100 will receive a trouble condition, and no alarm is sent.**
Both Zone 99 and 100 trigger: **An alarm will be sent through as a regular alarm.**
#### **This is useful for fire zones to be cross-referenced to double verify that a fire has been appropriately detected such as linking a CO and a Fire detector Zone.**
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