Features of the Onguard 800 System NT-420 Gas Module

  • Fully programmable from front panel
  • No potentiometers to adjust
  • Prevent sensor drift of display utilizing a dead band area
  • Three levels of alarm set points with relating relays
  • All user adjustments executed with two membrane switches
  • Real time digital display from 0 to 100 %LFL, PPM, or % input current
  • Real time digital display of 4-20mA current
  • Optional 4-20mA recorder output
  • Under range, over range, over current, fault annunciation
  • 24VDC open collector fault output, failsafe mode
  • Ability to beckon Allestec Alarm, Fault and Relay Module
Allestec NT420 Gas Module for the Onguard 800 Series Gas and Fire Control Panel

Onguard 800 System NT-420 Gas Module

The Allestec model 1457 NT420 Gas Module monitors industry standard 4-20mA output signals, allowing a real time digital display of the input current. The NT420 is connected through the 800 panel motherboard buss and is able to beckon the Alarm, Fault and the Relay Modules. This module is capable of being utilized as a standalone module, or can be integrated with other Allestec modules.

Programming is completed though the two front panel membrane switches. A sonalert responds to each command as the parameters are entered. While in the program mode, all relay outputs are disabled and the fault circuit is active. For each alarm level, the respective relay output will energize and follow its latching mode as established in the program setup. The program mode is also capable of displaying the actual 4-20mA loop current in real time. This feature can be utilized for loop current diagnostics for system servicing.

An optional 4-20mA output is available for a recorder or similar type of measuring instrument. This output signal is an exact duplicate of the input signal and has no influence on the module calibration.

Design Features

  • True analog to digital conversion
  • True digital display representation of loop current
  • Linear scale
  • Integral microprocessor design
  • Digital filter
  • Display digit fluctuating inhibitor
  • Memory retention with loss of power
  • 100 milliseconds loop current sampling rate
  • Peak hold of 4-20mA signal prevents display dither



Specifications of the Onguard 800 System NT-420 Gas Module


Approved operating voltage:

20 - 28 VDC power source

Operating current:

Quiescent with input = 4mA:85mA
Maximum alarm with input = 20mA:130mA

Dry relay outputs:

5 amps, 30 VDC resistive, 250 VAC
Output relays are sealed and contains an inert gas
Relays are selective for N.O. or N.C.

Sensor/transmitter output 24 VDC power:

Fused for 3 amps


Red seven segment displays:

Low scale displays "ur" for under range, then -10 to 00
High scale displays 1H for 100, the "or" for over range and "oc" for over current

Dominant wavelength:


Segment height:

.3” high

Display LEDs:

Red - HIHI indicates high high alarm
Red - HI indicates high alarm
Orange - LO indicates low alarm
Yellow - FAULT indicates fault condition



1.04"W X 3.46"H X 6.4"D


4.2 ounces


Ambient operating temperature:

0°F to 150°F
-17°C to 65°C
90% humidity non-condensing

Packaging and exposure:


Part Number Description


NT420 gas detection module %LFL


NT420 gas detection module PPM


NT420 analog input module %


System manual

Onguard 800 Series Gas and Fire Control Panel Components

Onguard 800 Series Gas and Fire Control Panel - A Modular Special Hazard Safety System

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