Does a 24 V circuit need to be intrinsically safe in Zone 1?
Abstract
Not necessarily intrinsically safe, but it must be protected. 24 V is not safe by itself: its current, capacitance and inductance can ignite gas. In Zone 1 every circuit and device needs an EPL Gb concept such as Ex ib, Ex db, Ex eb or Ex mb.
Not necessarily intrinsically safe, but it must be explosion protected. A 24 V circuit is not safe in Zone 1 just because the voltage is low: a 24 V supply can deliver enough current, and its capacitors and coils can store enough energy, to ignite a flammable gas. Every circuit and device in Zone 1 needs a recognised type of protection for EPL Gb; intrinsic safety is one option, flameproof, increased safety or encapsulated equipment are others.
Short answer
- Zone 1 requires equipment of EPL Gb or better (IEC 60079-14). There is no exemption for extra-low voltage, SELV or PELV circuits; those terms are about electric shock, not ignition.
- Ignition depends on the energy in a spark, not on voltage alone. At 24 V, the tables of IEC 60079-11 put the ignition limit in the IIC test mixture (hydrogen) at about 260 mA for a simple resistive circuit, and at about 0.46 µF for a capacitor charged to 24 V.
- Intrinsic safety (Ex ia or ib) is the usual choice for instruments, because it allows live work and needs no heavy enclosures. A loop fed from the safe area requires certified associated apparatus (barrier or isolator) and a documented loop check.
- Other concepts are equally valid: a 24 V solenoid valve can be
Ex mborEx db, a 24 V junction boxEx eb, a 24 V transmitterEx db. The wiring then follows the ordinary Ex installation rules, not the IS rules. - Ex ic is EPL Gc and is acceptable in Zone 2 only.
Why 24 V can still ignite gas
An electrical circuit ignites gas in two ways: by a spark when a conductor opens, closes or touches earth, and by a hot surface. Three things feed a spark:
- the source, limited by its voltage and the current it can deliver into a short circuit;
- capacitance, which dumps its stored energy into a short almost instantly;
- inductance, such as a relay or solenoid coil, which releases its stored energy as an arc when the current is interrupted.
An ordinary 24 V d.c. power supply can deliver several amperes into a short circuit. That is far above the ignition limits, especially for IIC gases.
| At 24 V, IEC 60079-11 tables with a safety factor of 1 (the ignition limit) | IIC | IIB | IIA |
|---|---|---|---|
| Resistive circuit: short-circuit current | 261 mA | 650 mA | 896 mA |
| Capacitive circuit: capacitance | 0.46 µF | 2.75 µF | 11.0 µF |
These values come from the spark test apparatus in the most easily ignited test mixture of each group. Intrinsically safe designs stay below them with a safety factor of 1.5 for levels ia and ib. For inductive circuits the curves approach a constant spark energy of 40 µJ for IIC, which a 24 V solenoid coil exceeds many times over. The ignition curves describe sparks; hot surfaces add a separate limit through the temperature class.
Example (illustrative values): A 24 V d.c. solenoid valve draws 0.5 A and its coil has an inductance of 1 H. The energy stored in its coil, ½ L I², is about 125 mJ. That is over three thousand times the 40 µJ inductive limit for IIC. The valve's own coil, wiring and terminals need explosion protection, whatever the supply voltage.
The options in Zone 1
| Approach | How it prevents ignition | Typical 24 V uses |
|---|---|---|
Intrinsic safety Ex ia, Ex ib | Energy limited by associated apparatus in the safe area (or protected by another concept) | Transmitters, switches, low-power solenoids, sensors |
Flameproof Ex db | Ignition contained and the flame cooled in the flamepaths | Transmitters, solenoid heads, junction boxes with relays |
Increased safety Ex eb | No sparks or hot spots in normal operation | Terminal boxes, solenoid coils certified Ex eb |
Encapsulation Ex mb | Sparking parts sealed in compound | Solenoid coils, small electronics |
Pressurisation Ex pxb, Ex pyb | Gas kept out by protective gas | Panels and analysers |
Choosing between them is a design decision, not a voltage decision. Intrinsic safety is attractive for instruments because it allows live maintenance and Zone 0 use (level ia), uses ordinary instrument cable within calculated limits, and avoids Ex d covers and glands. Its drawbacks are the limited power (often well under 1 W for a conventional loop), the need for barriers or isolators, and the loop documentation. A 24 V circuit that needs more power, such as a heater or a large solenoid, is usually built with another concept.
What to check when a 24 V loop is not intrinsically safe
If the field device is, for example, Ex db or Ex mb, the 24 V circuit is treated like any other power circuit in the hazardous area:
- The device certificate and marking must cover Zone 1 (EPL Gb), the gas group and the temperature class.
- Cable and cable entry devices follow IEC 60079-14 for that type of protection, for example certified Ex d glands on an Ex d enclosure.
- Electrical protection against overload and short circuit is needed as for any circuit, and the circuit must be isolated before the enclosure is opened in the hazardous area (IEC 60079-17); the live maintenance rules for intrinsically safe circuits do not apply.
- A 24 V circuit that is not intrinsically safe must never be connected to an IS device or share an IS cable or terminal row.
Common mistakes
- Treating SELV or "low voltage" as explosion protection. It protects people from shock; it does not limit spark energy.
- Feeding an
Ex iatransmitter straight from a 24 V supply. Without certified associated apparatus nothing limits the energy, so the loop is not intrinsically safe however low the voltage. - Forgetting inductance. Relay and solenoid coils store energy at 24 V; breaking their current is a classic ignition source.
- Using Ex ic in Zone 1. Level ic is EPL Gc, Zone 2 only.
Related on excodex
- Intrinsic safety (Ex i): ia, ib and ic and the energy limitation principle
- Intrinsic safety loop verification: Uo, Io, Po, Co, Lo and the entity concept
- Is a 4-20 mA transmitter automatically intrinsically safe?
- Which Ex protection concepts are allowed in Zone 1 and which only in Zone 2?
- Standards spaces: IEC 60079-11, IEC 60079-14
Standards and further reading
- IEC 60079-14, electrical installations design, selection and installation (edition 6, 2024), including EPL selection by zone.
- IEC 60079-11, equipment protection by intrinsic safety "i" (edition 7, 2023), with the reference ignition curves and tables.
- IEC 60079-25, intrinsically safe electrical systems.
- IEC 60079-11:2011 text (edition 6, superseded) with Tables A.1 and A.2 for permitted currents and capacitances, incorporated by reference in US federal regulations: law.resource.org
- IEC 60079-14:2024 preview with contents and table of significant changes: cdn.standards.iteh.ai
This article is general technical information prepared by the excodex editorial team. It does not replace the applicable standards, the equipment certificates and instructions, national regulations or the assessment of a competent person.