In Instrumentation, cables are broadly classified as IS (Intrinsically Safe) and Non-IS (Non-Intrinsically Safe) cables based on the type of electrical circuits they are connected to.
1. IS Cables (Intrinsically Safe Cables)
IS cables are used for intrinsically safe circuits, where the electrical energy is limited to prevent sparks or heat capable of igniting hazardous gases or vapors.
These are commonly used in hazardous areas such as:
- Oil & Gas plants
- Refineries
- Chemical plants
- Offshore platforms
Purpose
The purpose of an IS system is to ensure that even under fault conditions, the circuit cannot produce enough energy to cause an explosion.
Typical Applications
- Pressure transmitters
- Temperature transmitters
- Gas detectors
- Flow transmitters
- Field instruments in hazardous zones
Identification
IS cables are usually identified by:
- Light blue color outer sheath or marking
- Blue cable tags/glands/junction boxes
(Blue color is an international practice for IS systems.)
Characteristics
-
Connected through:
- IS barriers
- Galvanic isolators
- Low voltage and low current circuits
- Require segregation from non-IS cables
Installation Requirements
IS cables must:
- Be routed separately from non-IS cables
- Maintain minimum separation distance
- Use dedicated cable trays or partitions
- Follow hazardous area standards
Relevant Standards
- IEC 60079
- NFPA
- ATEX standards
2. Non-IS Cables (Non-Intrinsically Safe Cables)
Non-IS cables are used for normal electrical/instrumentation circuits where energy limitation is not provided.
These cables can carry enough electrical energy to ignite hazardous atmospheres if a fault occurs.
Typical Applications
- Control valves
- Solenoids
- Motors
- Marshalling signals
- Power supplies
- Non-hazardous area instruments
Characteristics
- Normal instrumentation/control cables
- No intrinsic safety barriers required
- Higher voltage/current possible
Identification
Non-IS cables usually:
- Do not have blue color identification
- Use standard cable markings
Key Difference Between IS and Non-IS Cables
| Feature | IS Cable | Non-IS Cable |
|---|---|---|
| Safety Concept | Energy limited | Normal energy |
| Hazardous Area Use | Yes | Limited/with protection |
| Color Identification | Light Blue | Standard colors |
| Requires IS Barrier | Yes | No |
| Explosion Protection | Intrinsic Safety | Other protection methods |
| Segregation Required | Strictly required | Normal routing |
Important Practical Point
In projects such as:
- DCS
- ESD
- F&G systems
IS and non-IS cables must never be mixed randomly in the same marshalling/JB/tray without proper segregation.
Typical site practices include:
- Separate cable trays
- Metallic partitions
- Minimum spacing
- Separate gland plates
Simple Example
IS Circuit
Pressure Transmitter → IS Barrier → DCS AI Card
Non-IS Circuit
Control Valve Solenoid → Marshalling Cabinet → DO Card
Common Cable Types Used
IS Instrument Cable
- 1 Pair / 2 Pair shielded twisted pair
- Overall shield
- Blue sheath
Non-IS Instrument Cable
- Multi-pair instrument cable
- Control cable
- Power cable
Easy Way to Remember
- IS = Safe energy level
- Non-IS = Normal energy level
Or:
IS prevents explosion by limiting energy.
Non-IS uses other protection methods or normal installation practices.
