What Is a Cause and Effect Diagram?

A cause and effect diagram (C&E diagram, also called a cause and effect matrix or C&E chart) is an engineering document that defines the logical relationship between input signals (causes) and output actions (effects) in a safety or control system. In oil and gas facilities, C&E diagrams specify what happens when a safety sensor detects an abnormal condition: which alarms activate, which valves close, which equipment trips, and which fire suppression systems engage.

The C&E diagram is the definitive logic document for Emergency Shutdown (ESD) systems, Fire and Gas (F&G) systems, and process safety shutdowns.

When C&E Diagrams Are Used

C&E diagrams are developed during detailed engineering, after the HAZOP study and SIL classification have identified the required safety functions. They translate safety requirements into programmable logic that is implemented in the Safety Instrumented System (SIS) or the Fire and Gas control panel.

C&E Element	Description	Example
Cause (input)	Sensor signal or condition that triggers action	PAHH on V-101 (pressure high-high)
Effect (output)	Action taken by the safety system	Close SDV-101, trip compressor K-101, alarm in control room
Logic	Voting arrangement or condition	2oo3 (2 out of 3 sensors must confirm)
Time delay	Intentional delay before action	5-second confirmed gas detection before ESD
Override/bypass	Conditions under which the action is inhibited	Maintenance bypass with key switch
Reset	How the system is returned to normal	Manual reset from control room after cause clears

C&E Matrix Format

The most common format is a matrix (spreadsheet) with:

• Rows: Input signals (causes)—each row represents a specific sensor or condition (e.g., PAHH-101, LAHH-102, Gas Detector GD-201)
• Columns: Output actions (effects)—each column represents a specific action (e.g., close SDV-101, trip pump P-201, activate deluge DV-301, sound alarm)
• Intersections: An “X” or symbol at the intersection means “this cause triggers this effect”

Cause (Input)	Close SDV-101	Trip K-101	Activate Deluge DV-301	Alarm Panel
PAHH V-101	X	X	-	X
TAHH E-101	-	X	-	X
LAHH V-102	X	-	-	X
Gas Det. GD-201 (confirmed)	X	X	X	X
Fire Det. FD-301 (confirmed)	X	X	X	X
Manual ESD pushbutton	X	X	X	X

This simplified example shows that a confirmed gas detection (GD-201) triggers all four effects: SDV closure, compressor trip, deluge activation, and alarm. A high-temperature alarm on the exchanger (TAHH E-101) only trips the compressor and raises an alarm, without SDV closure or deluge.

C&E Diagram Development Process

1. HAZOP: Identifies hazardous scenarios and required safety actions
2. SIL study: Assigns integrity levels to each safety function
3. C&E development: Safety/instrument engineer creates the C&E matrix based on HAZOP actions and P&ID safety annotations
4. Process review: Process engineering verifies that the logic correctly reflects the process safety requirements
5. Operations review: Operations team validates that the logic is practical and does not cause unnecessary shutdowns
6. SIS programming: The C&E matrix is the primary input for programming the safety PLC logic
7. FAT/SAT: The programmed logic is tested against the C&E matrix point by point

Common Mistake

Omitting the time delay logic from the C&E diagram causes nuisance trips during transient conditions. For example, a momentary gas detector spike from a brief, non-hazardous release (e.g., sampling operation) can trigger a full plant ESD if the C&E specifies immediate shutdown on any single gas detection. Best practice is to require confirmed detection (2oo3 voting or time-delayed single detector) before initiating high-consequence actions. Always define the voting logic and time delays explicitly in the C&E matrix, not just in the SIS programming code.

The C&E diagram is a critical bridge between the safety studies (HAZOP, SIL) and the physical implementation of safety systems. For more on how these documents fit into the overall engineering workflow, see the process documentation guide.

Read the full guide to piping engineering