OS vs IS/OS Instrumentation Cables: Which Shielding Do You Need?
In heavy industrial plants across India, engineers are careful about EMI. Cable trays are segregated. Data lines such as twisted pair cables carrying data, are kept away from power cables. The protocols are followed, and yet automated valves still misfire, PLC monitors flicker, and 4-20 mA sensors feed garbage data to the control room.
The problem is rarely outside the cable. Most of the time, it is inside it.
The Threat Nobody Accounts For: Crosstalk
To save space and installation time, contractors routinely bundle multiple pair cables inside one multi-core instrumentation cables. That is fine, as long as those pairs carry the same signal types. Otherwise, it leads to something called ‘crosstalk.’
Crosstalk happens when the electromagnetic field from one pair leaks into an adjacent pair inside the same cable.
For instance, if one pair carries a sharp digital pulse and the other runs a delicate 4-20mA analog signal, the digital pulse induces a voltage spike in the analog line. The reading gets corrupted, leading to downstream failures.
On a 4-20mA loop, noise above 0.16 mA peak-to-peak (less than 1% of the signal span) is enough to skew data. These loops are designed to run reliably over 1000 meters. Over those distances, even low-level interference accumulates.
The consequences are serious:
- In oil and gas, a corrupted pressure reading is not a data quality issue, it is a safety event. For instance, a control room reading “safe” when actual pressure is critical is on the verge of a catastrophic failure.
- In manufacturing, one hour of unplanned downtime in India typically costs Rs 2 lakh to Rs 15 lakh, depending on the sector.
And here is what catches most engineers off guard: A standard overall shield does nothing to prevent this. It only blocks interference from outside the cable. It does not protect the wire pairs from each other.
OS vs IS/OS: What’s what?
Overall Shielded (OS)
An OS cable wraps all the twisted pairs together in a single metallic shield, usually aluminium foil. These are also referred to as overall screened cables in some specifications.
This provides 100% surface coverage against external EMI. One shield, facing outward. But the pairs inside are unprotected from each other and vulnerable to crosstalk.
Use OS when: All pairs in the cable carry the same signal type. Identical digital signals, or low-sensitivity control signals, where crosstalk is not a meaningful risk.
Individually and Overall Shielded (IS/OS)
IS/OS cable, also called shielded twisted pair cable, adds a foil shield around each pair first, then wraps the whole bundle in a second outer shield.
Two layers: individual shields acting as internal firewalls between pairs, and the outer shield handling external EMI.
A sharp digital pulse in pair 1 stays in pair 1. The 4-20mA analog loop in pair 2 never sees it.
Construction standards for IS/OS cables are defined under IEC 60332, ICEA S-73-532, and ISA-5.1.
Specify IS/OS when:
- Mixed analog, digital, or communication signals share the same cable
- The cable carries millivolt-level signals like thermocouples and RTDs (below 100mV, extremely vulnerable to induced noise)
- Conductors run adjacent to VFD output cables carrying PWM switching power
- The installation is in an intrinsically safe area
- The application is in oil and gas, pharma, precision manufacturing, or power generation
Grounding: The Step That Makes or Breaks Your Shielding
Any reputable cable supplier will let you know that specifying IS/OS is only half the job. Grounding it correctly is equally important, in fact, critical. A shielded cable works only if it is correctly grounded.
A shield grounded at both ends creates a ground loop. Current flows through the shield between two ground references and generates its own interference. You have now built the exact problem you were trying to solve.
The solution:
- Ensure to ground only at one point and always at the control panel end.
- At the ungrounded field end, ensure individual pair shields are not touching each other or the overall shield.
Caution: Contractors experienced with power cables sometimes apply power cable grounding logic here. That is the mistake. Signal cable grounding is different.
Making the Right Call on Cable Specification
For simple, point-to-point signal runs carrying a single analog loop or one communication pair, a 2 core twisted pair cable with overall shielding is often sufficient.
The question of IS vs OS only becomes relevant when you move to multi-pair cables carrying mixed signals in the same jacket.
- Yes: OS is adequate and more economical.
- No: IS/OS is not optional. It is the only construction that solves the problem.
Getting this wrong during specification rarely shows up immediately. The early signs are intermittent: sensors that occasionally read oddly, valves that sometimes behave unexpectedly.
By the time the root cause is traced back to the cable, the troubleshooting hours and production losses have already cost far more than the upgrade would have.
Protect Your Signal Integrity: Partner with BMI Cables
Getting your signals right also depends on the partner you choose to manufacture your cables.
Reliable signal cables requires consistent construction quality, tight manufacturing tolerances, and a supplier who is available when your installation team hits a problem on site. This is where BMI Cables comes in.
Over 21 years of goodwill as leading instrumentation cable manufacturers in India means we have supplied instrumentation cables and shielded cables to every kind of industrial environment across India: Oil and gas, Power, Petrochemical, Pharmaceutical and Process manufacturing. Moreover, one of the trusted shielded cable manufacturers and instrumentation cable suppliers in the country, we are always available at your service
As part of the ESS ELL Cable Group and operating under ISO 9001:2015-certified processes, our OEM and EPC partners get:
- Advanced production infrastructure and consistent output
- In-house testing facilities for stringent quality validation
- Third-party certifications for added assurance
- Technical support on cable selection, installation, and grounding through every stage of the project
Our team is based out of New Delhi, with a state-of-the-art manufacturing facility in Bhiwadi, Rajasthan.
Frequently Asked Questions
OS has one shield around all pairs, blocking external EMI only. IS/OS adds a separate shield around each pair, preventing internal crosstalk between pairs.
Whenever you route different signal types in the same cable. If all pairs carry identical signals, OS is sufficient. In oil and gas, pharma, and precision manufacturing, IS/OS is the safer default.
No. It only blocks interference from outside the cable. Only individual pair shields prevent internal crosstalk between pairs inside the cable.
One end only, the control panel end. Grounding both ends creates a ground loop. At the field end, individual shields must be isolated from each other and from the overall shield.
Oil and gas, petrochemical, power generation, pharmaceutical, and food processing. Any sector where signal accuracy affects safety or unplanned downtime is expensive.