The Single Data-Bus Fault

When One Wiring Fault Can Affect Multiple Protective Functions

A significant vulnerability has been alleged—and, in some system configurations, demonstrated—in security and combination fire-and-burglar alarm systems that depend upon shared data-bus or auxiliary-power wiring.


Under ordinary conditions, these systems may arm, detect an alarm, operate local notification devices, and communicate with a central station as expected. A routine test may indicate that every device is working.


The concern arises when a short circuit or comparable fault occurs on shared field wiring.


Depending upon the control equipment, system architecture, wiring, programming, and communication arrangement, one fault may affect multiple functions simultaneously. Keypads, detection devices, expansion modules, local notification, and communication with the central station may be impaired precisely when the system is needed most.

What Is a Single Data-Bus Fault?

Many alarm systems use shared low-voltage wiring to connect the control panel with keypads, zone expanders, wireless receivers, communication modules, and other field devices. Separate auxiliary-power circuits may supply motion detectors, glass-break detectors, and other powered equipment.


A short circuit on either of these shared pathways can place excessive demand on the control panel’s power supply. Protective circuitry may then limit or interrupt DC power to prevent overheating or equipment damage.


In a properly isolated design, the effect may be confined to the affected circuit and reported as an actionable trouble condition.


In a more dependent architecture, the fault may impair several protective functions at once. The system may lose communication with connected devices, local warning may be reduced or unavailable, and alarm transmission may be interrupted until the fault is removed.


The exact response is system-specific. It should be established through equipment documentation, system review, and—when authorized—controlled, non-destructive evaluation.

How Can the Fault Occur?

A shared-circuit fault does not have to result from a deliberate attack.


It may occur accidentally when wiring is damaged by:

  • Fire or excessive heat
  • Water intrusion
  • Construction or renovation activity
  • Deterioration, abrasion, or improper termination
  • Rodents or other environmental conditions
  • Failure of a connected device
  • Damage in an attic, basement, ceiling space, wall, or common area


The same dependency may also create an intentional vulnerability when accessible field wiring can be deliberately faulted. Keypads, powered sensors, exposed cabling, exterior devices, and equipment located above accessible ceilings may provide potential points of access.


The important question is not merely how the fault begins. It is whether one accessible fault can propagate through the system and disable protection elsewhere.

Why This Matters

Local warning may be lost

If a fault interrupts power or communication needed by sirens, bells, keypads, or other notification equipment, occupants may not receive the warning they expect during a burglary, fire, or other emergency.

Central-station signaling may be affected

Some communication devices depend upon the same control panel, power source, or shared architecture. If those dependencies are impaired, an alarm or actionable trouble signal may not reach the central station.

Failure may not be obvious

A system affected by a severe DC-power or communication fault may be unable to produce its normal keypad indication, trouble sound, local alarm, or remote notification. The absence of a warning should not automatically be interpreted as normal operation.

Routine testing may not reveal the dependency

A conventional inspection normally confirms that individual devices operate under expected conditions. It may not determine what happens when shared wiring is shorted, auxiliary power is overloaded, communication is interrupted, or voltage falls outside the equipment’s reliable operating range.

Pending Federal Litigation

A putative federal class action filed in the United States District Court for the Northern District of Illinois—Martucci v. Underwriters Laboratories Inc. et al., Case No. 1:26-cv-01561—raises allegations concerning certain combination-listed, single-data-bus burglar and fire alarm control units tested and listed by Underwriters Laboratories entities.


The complaint alleges that certain systems contain defects affecting their performance during data-bus or auxiliary-power faults and that the systems were represented as compliant with applicable UL and NFPA 72 standards. The complaint discusses products associated with Honeywell/Resideo, Johnson Controls/DSC, and Digital Monitoring Products.


These are allegations in pending litigation. They are contested and have not been finally adjudicated.


The lawsuit is important because it places greater attention on a broader technical question:



Can one fault on shared alarm-system wiring prevent local warning or central-station signaling during the event the system was intended to detect?

An Important Distinction

The litigation does not establish that every model produced by a named manufacturer is affected. It also should not be interpreted to mean that products or manufacturers not named in the complaint are necessarily free from similar architectural dependencies.


Comparable fault behavior has been publicly discussed or demonstrated in other equipment. Those products and manufacturers may not be parties to the case.


For that reason, SSI does not evaluate susceptibility solely by brand name, product listing, or involvement in litigation. We examine the installed equipment, wiring, power distribution, programming, communication paths, supervision, and response procedures of the individual system.

What Owners Should Determine

Property owners and responsible organizations should be able to answer several practical questions:

  • What happens when the system’s data-bus wiring is shorted?
  • What happens when auxiliary DC power is faulted?
  • Is the condition isolated, or can it affect the entire system?
  • Does the control panel generate a specific and actionable event?
  • Will an appropriate local warning remain available?
  • Will the central station receive enough information to initiate the correct response?
  • Are communication devices and remote power supplies independently powered and supervised?
  • Can accessible wiring be protected, relocated, isolated, or segmented?
  • Has the system’s actual behavior been observed, or is its performance being assumed?

What Can Be Done?

Do not wait for a failure to reveal the dependency

Many owners have never been shown how their systems behave during shared-circuit faults, abnormal power conditions, or communication failures. Proactive evaluation can identify material dependencies before an emergency or loss occurs.

Request an Alarm Resilience Assessment

Security Systems Inc. provides structured, non-destructive evaluations of selected fault, power, communication, supervision, and tamper conditions.

An assessment may include:

  • Review of the control equipment and system architecture
  • Identification of shared wiring and power dependencies
  • Evaluation of accessible fault and tamper exposure
  • Observation of local system behavior
  • Review of central-station signals and response instructions
  • Examination of reserve power and communication dependencies
  • Documentation of observed limitations and corrective options



An assessment does not certify that a system cannot be defeated or guarantee against loss. Its purpose is to replace unsupported assumptions with system-specific information.

Correct the condition according to the findings

Depending upon the equipment and installation, corrective options may include:

  • Relocating or protecting accessible wiring
  • Separating or segmenting critical circuits
  • Adding appropriate fault isolation
  • Revising power distribution
  • Using supervised remote power supplies
  • Providing independent or more resilient communication paths
  • Reprogramming actionable fault reporting and response procedures
  • Replacing equipment when the existing architecture cannot provide an acceptable level of resilience



The appropriate remedy depends upon the system and the protection objectives of the owner. No single modification is suitable for every installation.

Protection Should Be Demonstrated, Not Assumed

A security system should not be judged only by how it performs when conditions are normal.


The more important question is what remains operational when wiring is damaged, power is disrupted, communication is lost, or an accessible component is deliberately faulted.


Know how the system performs before a failure reveals it.