Dettaglio News

During the loading or unloading of a tanker truck, drum, or IBC in an ATEX zone, earthing is the only barrier standing between a routine operation and an accidental ignition. Yet not all earthing clamps offer the same level of protection — and the difference isn’t in the price, it’s in how they work.

In this article, we look at what separates a “signal-only” earthing clamp from one with a SIL2 interlock, what the regulations require, and how to determine — when choosing equipment or during an audit — whether your plant is genuinely covered.

In short: a SIL2 interlock on an earthing system is a safety device that physically prevents a transfer operation from starting in an ATEX zone (Zone 1/21) if the electrical continuity of the earth connection has not been verified — unlike a signal-only system, which merely alerts the operator without stopping the operation.

The problem: a warning light isn’t a barrier

Many earthing clamps simply signal the connection status through an indicator light or a signal contact. They work like this:

• the operator connects the clamp to the earthing point
• an LED (or a panel signal) indicates whether contact has been made
• starting the transfer remains a manual decision made by the operator

The limitation is obvious: safety depends on human attention. If the operator starts the transfer without checking the light, or if the connection loosens during the operation, the system flags the problem but doesn’t stop it. In a classified area, this means the only real barrier against electrostatic charge accumulation can be bypassed by a simple moment of distraction.

Interlock vs. signal-only: what’s the difference?

A system with an interlock contact does more than signal a fault: it physically prevents the operation from starting if earthing hasn’t been electrically verified, whereas a signal-only system alerts the operator but leaves the final call to them. In practice, an interlocked system:

• measures the actual electrical continuity of the earth connection, not just the physical presence of the clamp
• withholds permission to start the transfer (pump, loading valve) until the reading falls within the safety threshold
• automatically interrupts an operation already in progress if the earth connection is lost, not only at start-up

The difference between the two approaches is conceptual before it’s technical: a signal-only system informs the operator, while an interlocked system decides on their behalf when safety isn’t guaranteed.

Why SIL2, and not just any interlock

Not all interlocks offer the same level of reliability. SIL (Safety Integrity Level) is the classification, defined by the IEC 61508 standard, that measures how reliable a safety function is — in other words, how low the probability is that the system will fail exactly when it’s needed.

An interlock contact certified to SIL2 provides a level of reliability appropriate for industrial safety functions where failure could have serious consequences — which is exactly the case for a transfer operation in a classified area. Choosing it isn’t excessive caution: it’s matching the actual risk of the operation to the required level of protection.

It’s not by chance that our range no longer includes signal-only solutions: a system that can only inform, without being able to stop the operation, leaves a margin of risk we consider unacceptable in a classified area.

What the regulations require

The ATEX Directive 2014/34/EU sets the safety requirements for equipment intended for use in areas with an explosion hazard — including earthing systems installed in those areas. But there’s a second, often less closely watched, regulatory layer: Directive 1999/92/EC, which requires employers to classify hazardous zones and document the prevention measures adopted in the Explosion Protection Document (EPD). We have examined the differences between the two directives, and the respective responsibilities, in this article.

During an audit or inspection, it isn’t enough to show that you have “an earthing clamp”: you need to demonstrate that the system chosen matches the risk level of the classified zone, and that the safety function is verifiable, not just declared on paper.

Checklist: what to check before choosing (or during an audit)

• Does the system only signal, or does it physically block the start of the operation when earthing isn’t verified?
• Does the interlock contact carry a declared SIL certification (and at what level)?
• Does the system detect a loss of connection during the operation, not only at start-up?
• Is the device’s ATEX marking consistent with the installation zone (gas/vapor or dust, category and group)?

The Logic solution

Logic has been designing earthing systems and ATEX equipment for over 40 years, with installations across the chemical, cosmetics, and pharmaceutical industries. Our ECS-RF earthing clamps come standard with a SIL2 interlock contact, certified under ATEX Directive 2014/34/EU for use in Zone 1/21 (up to Zone 0/20 depending on configuration). The system electrically verifies the continuity of the earth connection and withholds permission to start the transfer until earthing is confirmed — covering tanker truck loading bays as well as systems for drums, IBCs, and big bags.

If you need to verify the compliance of an existing plant, or you’re evaluating a new earthing system for a transfer line, get in touch with us for a risk assessment specific to your process.