A recurring objection in Russian and CIS projects: the specification says flameproof, the robot is pressurized, and the safety engineer stops the review. The two are both recognised protection types under the same standards. They solve the problem in opposite ways, and for a moving six-axis arm inside a solvent atmosphere only one of them is practical.
Flameproof (d) — contain the explosion
A flameproof enclosure accepts that the explosive atmosphere gets inside and is designed so that an internal ignition cannot propagate outside: heavy walls, controlled flame paths at every joint. It is proven, well understood, and it is why buyers with a process-plant background write 1ExdIIC into a specification almost automatically.
On a robot arm the approach breaks down. Every joint would need a certified flame path, the mass penalty is enormous on a moving structure, and the cable and paint-line entries multiply the number of critical joints.
Pressurization (px) — keep the atmosphere out
Pressurization keeps the interior of the arm and motors above the booth pressure with clean protective gas, so the flammable atmosphere never enters. This is what the GR and SR series use: Ex pxb, combined on the SR series with increased safety (e), intrinsic safety (i) and encapsulation (m) on specific components.
The price of this approach is that protection depends on a live utility. Pressurization is only valid while the gas supply holds, which is why the certified system includes a pre-purge, a pressure monitor and an interlock that removes power when pressure is lost.
What the installation must provide
- A clean, dry protective gas supply — instrument-quality air or nitrogen — sized for both purge flow and leakage compensation.
- A pre-purge before energizing: on the SR series, 15 minutes at 0.05–0.15 MPa before the drives can be powered.
- Continuous pressure monitoring with an interlock that de-energizes the arm on loss of pressure, and a defined restart sequence that repeats the purge.
- The control cabinet outside the hazardous zone (IP54 as supplied), with certified cable entries where the loom passes the booth wall.
- Ambient conditions within the certificate: the SR series is certified for 0–45 °C.
Answering the flameproof objection
The productive answer is a standards comparison table, not an argument. Set out, side by side, the zone your booth is classified as, the protection types accepted for that zone under the applicable standard, the marking your robot carries, and the conditions the installation must meet. A safety engineer is being asked to sign; give them the document that lets them sign.
We now put that table in the first proposal for any project where the specification mentions flameproof equipment, rather than waiting for the objection.
Frequently asked questions
Is pressurization less safe than flameproof?
Neither type is inherently safer — both are recognised for the same zones when correctly applied. They differ in what they depend on: flameproof depends on mechanical integrity, pressurization on a monitored gas supply with interlocks. For a moving robot arm, pressurization is the practical option.
What happens if the purge air fails mid-cycle?
The certified system removes power from the arm and signals the fault. Restart requires the full pre-purge again — this is not a nuisance to be bypassed; it is the mechanism the certificate relies on.
Can I use plant compressed air for purging?
Only if it meets the quality the manufacturer specifies — oil and water carry-over defeat the purpose and damage the motors. Where the plant air is uncertain, we quote the treatment package with the robot rather than leaving it as a site item.
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