Intrinsic Safety

What is intrinsic safety?

Intrinsic safety (Ex i) is a type of protection for electrical equipment in explosive atmospheres that keeps the electrical and thermal energy in a circuit so low that no spark or hot surface can ignite the atmosphere, even under fault conditions. Rather than containing an explosion inside a heavy enclosure, Ex i ensures there is never enough energy to start one. That is why intrinsic safety is the default choice for instrumentation in an hazardous area: transmitters, sensors, valve position feedback and HART communication in chemicals, oil and gas, and pharmaceuticals.

🧠 How does intrinsic safety work?

Every flammable gas or dust has a minimum ignition energy. For gas group IIA (reference gas propane) it is around 0.25 mJ, for IIB (ethylene) around 0.1 mJ and for IIC (hydrogen, acetylene) around 0.02 mJ. An intrinsically safe circuit therefore limits three things:

• Voltage and current — Zener diodes and resistors cap the energy of a spark when a wire shorts or breaks
• Stored energy — capacitance and inductance in the device and in the cable can discharge all at once during a fault
• Surface temperature — components must stay below the temperature class (for example T4 = 135 °C maximum)

An Ex i system always has two parts: the intrinsically safe apparatus in the field and the associated apparatus, usually a barrier or isolator in the safe area. You can recognise the latter by the square brackets in its marking, such as II (1)G [Ex ia Ga] IIC.

The idea is more than a century old. After the Senghenydd colliery disaster in Wales on 14 October 1913, which killed 440 people, the official investigation concluded that sparks from a bare-wire signalling bell had most likely ignited the firedamp. The principle of energy limitation grew out of that finding, first in mining and later in the process industries.

📜 Which standards apply to Ex i?

Intrinsic safety is part of the IEC 60079 series, adopted in Europe and the Netherlands as NEN-EN-IEC 60079:

The previous EN 60079-11:2012 is to be withdrawn by 31 December 2027, so manufacturers would be wise to update their certificates to the new edition in good time. Edition 7 tightens the rules on encapsulation, conformal coating and lithium-ion batteries, among other things.

🧱 What is the difference between ia, ib and ic?

The level of protection states how many faults a circuit can tolerate without causing ignition. For ia and ib the standard also applies a safety factor of 1.5 to the spark energy; for ic the factor is 1.0.

The EPL (Equipment Protection Level) links the level to the ATEX equipment category: Ga corresponds to category 1G, Gb to 2G and Gc to 3G. Ex ic was added to IEC 60079-11 in 2006 as a simpler option for Zone 2, replacing the older energy-limited protection method Ex nL.

🔧 How do you verify an intrinsically safe loop?

Under the entity concept, you compare the output parameters of the barrier with the input parameters of the field device, plus the cable. You will find all values on the certificates and rating plates.

Worked example — a 4-20 mA pressure measurement in Zone 0, gas group IIC:

1. Barrier: Uo = 28 V, Io = 93 mA, Po = 0.65 W, Co = 83 nF, Lo = 2 mH (common values for a 28 V barrier in IIC)
2. Transmitter (example): Ui = 30 V, Ii = 100 mA, Pi = 0.75 W, Ci = 10 nF, Li ≈ 0
3. Voltage, current, power: 28 ≤ 30 V, 93 ≤ 100 mA, 0.65 ≤ 0.75 W — pass
4. Cable: 300 m at 100 pF/m and 0.7 µH/m gives 30 nF and 0.21 mH
5. Capacitance: 10 + 30 = 40 nF ≤ 83 nF — pass
6. Inductance: 0 + 0.21 = 0.21 mH ≤ 2 mH — pass

Here capacitance sets the maximum cable length: (83 − 10) / 0.1 ≈ 730 m. Watch the 1% rule: if both Ci and Li exceed 1% of Co and Lo, you may only use half of Co and Lo. In this example Li is negligible, so that restriction does not apply. Record the calculation in the loop file and reference it from the P&ID, so that a future transmitter replacement is checked again.

🔄 Zener barrier or galvanic isolator?

Modern installations usually opt for isolators: they need no dedicated earthing system and are less error-prone when the plant is extended.

🏭 What installation requirements apply to Ex i?

Beyond the loop calculation, IEC 60079-14 sets several practical requirements:

• Light blue marking — if cables, trays or terminals are colour-coded, the colour is light blue
• Separation — terminals of intrinsically safe and non-intrinsically safe circuits are kept 50 mm apart or divided by a partition
• No mixing — Ex i circuits do not share a cable with other circuits
• Documentation — a descriptive system document with calculations, certificates and drawings
• Live working — under defined conditions you may work on intrinsically safe circuits inside the hazardous area without isolating them, a major advantage for calibration

🌐 How does Ex i work with fieldbuses and Ethernet-APL?

For fieldbuses such as Foundation Fieldbus H1 and Profibus PA, with many devices on one cable, the entity calculation becomes laborious. The German PTB therefore developed FISCO (Fieldbus Intrinsically Safe Concept): a single power supply with Uo between 14 and 17.5 V, cable within fixed limits (15–150 Ω/km, 0.4–1 mH/km, 45–200 nF/km), a total cable length of up to 1 km and spurs of up to 60 m. FISCO was laid down in IEC 60079-27 in 2005; since 2011 its requirements have been part of IEC 60079-11 (apparatus) and IEC 60079-25 (systems).

Ethernet-APL uses the comparable 2-WISE concept from IEC TS 60079-47 (February 2021). Each port has fixed Ex parameters; if the port profiles of switch and instrument match, the spur is intrinsically safe right into Zone 0 with no calculation needed. The trunk between switches is not intrinsically safe but uses increased safety (Ex e or Ex ec) for Zone 1 or 2. Built on 10BASE-T1L, Ethernet-APL is the process-industry variant of Single Pair Ethernet, suitable for hazardous areas.

🔄 How does Ex i compare with Ex d, Ex e and Ex p?

Ex i is the only one of these four that reaches Zone 0 and allows maintenance on live circuits, but it only works for low power levels, in practice usually around 1 W or less.

🇳🇱 What do you need to arrange in the Netherlands?

Manufacturers certify Ex i equipment under ATEX 114 (2014/34/EU). Employers fall under ATEX 153 (1999/92/EC), implemented in the Netherlands in articles 3.5a to 3.5f of the Arbobesluit under the Arbowet. These require an explosion protection document (explosieveiligheidsdocument) that records the zone classification and the chosen measures. After commissioning, IEC 60079-17 requires periodic inspection (visual, close or detailed), at intervals of no more than three years unless an expert justifies otherwise.

🔐 Does cybersecurity matter for intrinsically safe equipment?

Yes. Intrinsic safety protects against ignition, not against tampering. An Ex ia transmitter often has a digital interface through HART, WirelessHART or Ethernet-APL. Anyone who gains access through a HART multiplexer or asset management software can change measuring ranges, damping or software write protection. So use the hardware write protection (jumper or switch) in the instrument where available, segment the network that reaches your instrumentation and include field instruments in your risk assessment.

❓ Frequently asked questions

What does Ex ia mean on a rating plate?

Ex ia means that the device is intrinsically safe at the highest level of protection and cannot cause ignition even with two simultaneous faults. Ex ia equipment has EPL Ga and may therefore be used in Zone 0. A marking such as Ex ia IIC T4 Ga also states the gas group and temperature class.

Can an intrinsically safe transmitter be connected to any barrier?

No, an intrinsically safe transmitter may only be connected to a barrier or isolator whose Uo, Io and Po do not exceed the transmitter’s Ui, Ii and Pi. The capacitance and inductance of the transmitter plus cable must also stay within the barrier’s Co and Lo. This check is called the entity calculation.

Why does a Zener barrier need a dedicated earth?

A Zener barrier diverts excess fault current through its Zener diodes to earth, so that no dangerous energy reaches the hazardous area. That protection only works with a reliable, dedicated intrinsically safe earth of no more than 1 Ω to the main earth. A galvanic isolator does not need this earth.

What colour are intrinsically safe cables?

Intrinsically safe cables and terminals are marked light blue under IEC 60079-14 whenever colour coding is used. This makes intrinsically safe circuits instantly recognisable in a cabinet or cable tray. Light blue is therefore reserved for Ex i and is not used for other circuits.

Is intrinsic safety the same as functional safety?

No, intrinsic safety prevents an electrical circuit from igniting an explosive atmosphere, whereas functional safety is about reliably performing a safety function, such as a trip performed by a SIS. An instrument can have both at once, for example Ex ia and SIL 2. The standards and calculations are entirely separate.

Is Ethernet-APL intrinsically safe?

Ethernet-APL can be made intrinsically safe using the 2-WISE concept from IEC TS 60079-47. Spurs with an Ex ia port profile reach into Zone 0, while the trunk, protected by increased safety (Ex e), runs into Zone 1. Intrinsic safety with Ethernet-APL needs no loop calculation as long as the port profiles match.

📌 In summary

Intrinsic safety (Ex i) makes instrumentation in hazardous areas safe by limiting circuit energy so that ignition is impossible, with ia for Zone 0, ib for Zone 1 and ic for Zone 2. A correct loop calculation, the right barrier or isolator, careful installation and periodic inspection under IEC 60079-14 and -17 keep that protection intact throughout the plant’s life.