You're right to be thinking carefully about this. Combining multiple boilers under a single E-Stop system requires careful consideration to ensure safety and reliability. Here's a breakdown of approaches and recommendations

Why Directly Connecting Shunt Trip Coils is Generally a Bad Idea
Voltage/Current Mismatch

Shunt trip coils are designed for specific voltage and current. Connecting them in parallel might work if they are identical models with identical voltage and current requirements. However, even then, you're relying on perfect matching, which is unlikely in the real world. Even small variations can cause one coil to hog the current and prevent the others from tripping. Connecting them in series might work but voltage drop across the coils could prevent them from tripping.

Fault Propagation

If one shunt trip coil fails (e.g., burns out, shorts), it could potentially disable the entire E-Stop circuit. This is a single point of failure.

Backfeeding

Depending on the wiring, there's a risk of backfeeding voltage from one shunt trip circuit to another if one of the boilers is still running/has residual voltage.

Unknown Performance

You're venturing into territory where the behavior of the shunt trip breakers is no longer guaranteed or certified. They are designed to work with a single, defined load.

The Recommended Solution: Using a Contactor

Using a contactor is the best practice and safest approach for implementing a multi-boiler E-Stop system. Here's why: Isolation and Control

The contactor provides electrical isolation between the E-Stop circuit and the individual boiler power circuits. The low-voltage E-Stop circuit controls the contactor coil, and the contactor's high-current contacts interrupt the power to the boilers.

Scalability

You can easily add more boilers to the E-Stop system by adding more contactors controlled by the same E-Stop circuit.

Defined Performance

Contactors are designed for switching large loads and are rated for specific voltages and currents. You can select a contactor that is appropriately sized for your boiler loads.

Safety

Using a contactor allows for a clean and well-defined safety circuit that is easier to troubleshoot and maintain.

How to Implement the Contactor-Based E-Stop System1. E-Stop Buttons

Install appropriately rated and labeled E-Stop buttons at strategic locations. Wire the E-Stop buttons in series to create a normally closed (NC) E-Stop circuit. When any E-Stop button is pressed, the circuit opens.2.

Safety Relay/E-Stop Relay (Highly Recommended)

Consider using a dedicated safety relay or E-Stop relay in the E-Stop circuit. These relays are designed for safety-critical applications. They have features like:

Forced-guided contacts

Ensures that if a contact welds shut, other contacts won't move, providing a positive indication of a failure.

Redundancy

Many safety relays have redundant internal circuitry for increased reliability.

Self-monitoring

Constantly checks the E-Stop circuit for faults.

Certifications

Meet stringent safety standards (e.g., IEC 61508, EN ISO 13849). The E-Stop buttons are wired to the inputs of the safety relay.3.

Contactor Coil

The output of the safety relay (or directly from the E-Stop circuit if you don't use a safety relay, but this is less desirable) energizes the coil of the contactor. When the E-Stop circuit is closed (no button pressed), the contactor coil is energized, and the contactor contacts are closed. Select a contactor coil voltage that matches the output of your safety relay or E-Stop circuit.4.

Contactor Contacts

Each boiler should have its own contactor. The contactor contacts are wired in series with the power supply to each boiler. When the E-Stop button is pressed, the E-Stop circuit opens, the safety relay (if used) de-energizes, the contactor coil de-energizes, and the contactor contacts open, cutting off power to the boiler. Size the contactor contacts appropriately for the full load current of the boiler.5.

Shunt Trip Breakers (Optional, but Often Recommended)

Consider keeping the shunt trip breakers on each boiler as a secondary safety mechanism. The contactor provides the primary E-Stop function, and the shunt trip breakers provide additional protection in case of a contactor failure or other fault. If you use shunt trip breakers, you can wire them to a separate set of contacts on the safety relay (if it has them) or to a separate relay controlled by the E-Stop circuit.

Wiring Diagram Example (Simplified - Consult an Electrical Engineer for Proper Design)

+-----------------+| Power Source    |+--------+--------+|+--------v--------+| E-Stop Buttons  |  (Multiple in Series)+--------+--------+|+--------v--------+| Safety Relay   |  (Optional, Highly Recommended)+--------+--------+|+--------v--------+| Contactor Coil  |+--------+--------+|+--------------------+--------+--------+--------------------+|                    |                    |                    |+-------v-------+    +-------v-------+    +-------v-------+    +-------v-------+| Contactor     |    | Contactor     |    | Contactor     |    | Contactor     | (Boiler N)| Contacts      |    | Contacts      |    | Contacts      |    | Contacts      |+-------+-------+    +-------+-------+    +-------+-------+    +-------+-------+|                    |                    |                    |+-------v-------+    +-------v-------+    +-------v-------+    +-------v-------+| Boiler 1      |    | Boiler 2      |    | Boiler 3      |    | Boiler N      |+---------------+    +---------------+    +---------------+    +---------------+

Important Considerations



Redundancy

Consider adding redundancy to the E-Stop circuit. For example, you could have two E-Stop buttons in series on each boiler.

Testing

Regularly test the E-Stop system to ensure it functions correctly. Document your testing procedures and results.

Lockout/Tagout (LOTO)

Always follow proper LOTO procedures before performing any maintenance or repairs on the boilers or the E-Stop system.

Professional Design

This information is for guidance only. A qualified electrical engineer should design and implement the E-Stop system to ensure it meets all applicable safety standards and codes. They will be able to perform a hazard analysis, select the appropriate components, and create a complete and accurate set of drawings.

Local Codes and Standards

Ensure your design complies with all applicable local electrical codes and safety standards (e.g., OSHA, NFPA 70, etc.).

Control Reliability

If the application is safety-critical, consider using control-reliable components and systems. Control reliability means that the system is designed to fail in a safe manner.

SIL Level (Safety Integrity Level)

Depending on the risk associated with boiler operation, a SIL level may need to be defined for the safety functions. This will influence component selection and system design.

Documentation: Maintain complete and accurate documentation of the E-Stop system, including wiring diagrams, component specifications, and testing procedures.In summary, while directly connecting shunt trip coils might seem simpler, using contactors (preferably controlled by a safety relay) is the safest, most reliable, and most scalable approach for implementing a multi-boiler E-Stop system. Always consult with a qualified electrical engineer for proper design and implementation. Flag for review