“## Surge protection

When to use surge protection#

We recommend surge protection devices (SPDs) for the RS-485 bus in these cases:

• The bus is run outdoors
• The bus passes through areas outside the lightning protection system’s protection zone

How it works#

Surge protection, unlike a bus isolator, channels the induced voltage to the PEN/PE terminal, thereby protecting the input circuitry of your devices.

Power supply protection#

Power supply in a protected area#

When using the circuit shown in the attached diagram, there is no need to protect the positive pole of the power supply if the supply is located in a protected area.

Power supply in an unprotected area#

If the supply is located in an unprotected area, you need to use:

• Surge protection for the 24 V DC distribution
• Type C surge protection on the AC supply side

The actual design of the surge path depends on the cross-section of the incoming low-voltage distribution cable and the possibility of grounding the system.

Circuit diagram#

In the attached diagram, surge protection is used only on the supply line to the remote section located outdoors.

Warning: This is only a partial solution for protecting the bus. For full protection, see the section below.

Complete bus protection#

A complete protection solution requires two surge protection devices in the following cases:

When full protection is required#

The bus passes through sections that:

• Are, and then are not, within the protection zone of lightning protection systems
• Do not have the ability to absorb the energy field created by atmospheric discharges

Protection placements#

Surge protection devices are installed at the entry points of the conductors into protected zones.

Important: This applies regardless of whether an optical isolator is used on the bus.

Modbus bus protection#

When to use#

For a Modbus bus run outdoors outside the lightning protection system’s protection zone, we recommend using surge protection.

Power supply protection#

When using the circuit shown in the attached diagram, the positive pole of the power supply does not require protection, as it is not used.

Standards requirements#

When designing the system correctly, the risk of electromagnetic impulses from atmospheric discharges must not be neglected, in accordance with the STN EN 62 305 standards.

Circuit diagram#

The attached diagram shows complete protection of the Modbus bus line, equipped with surge protection on both sides of the bus conductors.

Surge protection for temperature measurement#

Measurement with resistance sensors (Pt100)#

When measuring temperature using resistance sensors (e.g., Pt100), consider:

• The ohmic resistance of the extension cables
• The damping resistors of protective devices

Effect on measurement accuracy#

Pay attention to measurement accuracy!

In a two-wire measurement, the SPD resistance can distort the measured result.

Example error:

• Sum of damping resistors: 4 Ω
• Measurement error at 0 °C: 4% (104 Ω instead of 100 Ω)

Solution#

Two-stage protective circuits are available in a version without damping resistors to minimize the SPD’s impact on measurement accuracy.

Surge protection for current loops#

4–20 mA signal#

Measured values are transmitted using the standardized 4–20 mA signal, which is used mainly for applications with longer conductors.

Advantages of current transmission#

• The cable’s resistance does not affect the current conveying the measured value
• Two signal wires are used
• No additional reference potential is required
• They are carried in an isolated state from earth potential

Protection#

Protection is required at both ends!

To protect this type of application from surges, SPDs are required at both ends of the conductors.

SPD construction#

The relevant SPD is equipped with a multi-stage protection circuit, which provides protection against:

• Transient voltage in normal operation between signal conductors
• Common-mode voltage to earth at both ends”