--- title: "Surge Protection" description: "When and how to protect RS-485 and Modbus buses run outdoors: surge protectors to PE, 24 V DC and T2 supply protection, Pt100 sensors and 4–20 mA loops." url: "https://taphome.com/en/docs/wiring/surge-protection/" language: en lastmod: 2026-10-08 generator: hugo ai: topics: ["surge-protection","rs-485-bus","modbus-bus","lightning-protection","power-supply-protection","temperature-measurement","current-loop"] questions: ["When should I use surge protection for RS-485 bus?","Where should surge protection devices be placed on the bus?","What type of AC protection is needed when power supply is in unprotected area?","Why do I need two surge protection devices for complete bus protection?","How much measurement error do SPD damping resistors cause for Pt100 sensors?","How do I protect 4-20 mA current loop sensors from voltage surges?","What do the EN 62305 and EN 61643-21 standards cover?"] --- # Surge Protection Essential surge protection for RS-485 and Modbus buses in exterior areas. Includes protection for power supplies and current loops. Prevents voltage damage with multi-stage circuits. Last updated: 2026-10-08 ### When to use surge protection Surge protection (voltage surge protector) for the RS-485 bus is recommended in these cases: - The bus is run through the **exterior** - The bus passes through an area **outside the protective zone of the lightning protection system** ### How it works Surge protection, unlike a bus isolator, diverts the induced voltage to the **PE terminal** and thus protects the input circuits 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 terminal of the power supply** if the source is located in a protected area. #### Power supply in an unprotected area If the source is located in an unprotected area, the following are required: - **Surge protection for the 24 V DC distribution** - **Type 2 (T2) surge protection on the AC input side** (formerly class C) > [!NOTE] > The actual down-conductor design depends on **the cross-section of the LV distribution feeder cable** and the possibility of **grounding the system**. ### Wiring diagram In the attached diagram, surge protection is used only on the supply to the remote section located outdoors. > [!WARNING] > **Warning:** This is only a **partial solution** for protecting the bus. For complete protection, see the section below. ![Surge protection on the remote section with an optical isolator and two power sources](https://taphome.com/documentation/wiring/optical-isolation-repeater-dual-power.svg) ### Complete protection of the bus A fully correct protection solution requires **two surge protection devices** in the following cases: #### When is full protection required The bus passes through sections that: - **Are and subsequently are not** in the protective zone of lightning protection systems - Do not have the ability to **absorb the energy field** created by atmospheric discharges #### Placement of protectors Surge protection devices are installed at the **entry points of conductors into protected zones**. > [!WARNING] > **Important:** This applies **regardless** of whether an optical isolator is used on the bus. ![Surge protection with separated lines](https://taphome.com/documentation/wiring/optical-isolation-dual-repeater.svg) ### Modbus bus protection #### When to use For a Modbus bus run through **exterior areas outside the protective zone of the lightning protection system**, we recommend using surge protection. #### Power protection When using the circuit shown in the attached diagram, there is no need to protect the positive terminal of the power supply since it is not used. #### Standards requirements > [!NOTE] > When designed properly, one should not neglect the risk of electromagnetic impulses from atmospheric lightning discharges in accordance with the EN 62305 series of standards. Surge protective devices for data and signal lines, such as RS-485, are tested according to EN 61643-21 (categories such as C2 and D1, not the power-system types 1, 2 and 3). #### Wiring diagram The attached diagram shows the **full protection of the Modbus bus line**, equipped with surge protection on **both ends** of the bus conductor. ![Surge protection Modbus](https://taphome.com/documentation/wiring/optical-isolation-modbus-repeater.svg) ### Surge protection for temperature measurement #### Measurement using resistance sensors (Pt100) When measuring temperature using resistance sensors (e.g. Pt100), consider: - **The ohmic component of the supplementary cables** - **Damping resistors of protective devices** #### Impact on measurement accuracy > [!WARNING] > **Pay attention to measurement accuracy!** > > In the case of two-wire measurement, the resistance value of the SPD can distort the measured result. > > **Example of error:** > - Sum of damping resistors: 4 Ω > - Measurement error: approx. 10 °C (at 0 °C the sensor reads 104 Ω instead of 100 Ω; a Pt100 changes by about 0.385 Ω per 1 °C) #### 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 especially in applications with longer conductors. #### Advantages of current-loop transmission - The cable’s ohmic resistance does not affect the current carrying the measured value - Two signal conductors are used - No additional reference potential is required - They are carried in an isolated state from the earth potential #### Protection > [!WARNING] > **Protection at both ends is required!** > > To protect this type of application from transients, an SPD is required at both ends. #### SPD construction The respective SPD is equipped with a multi-stage protective circuit that provides protection against: - Transient voltages in normal mode (differential mode) between signal conductors - Common-mode voltage to ground at both ends **See also** — [Modbus RTU Wiring Examples](https://taphome.com/en/docs/wiring/modbus-wiring/), [Optical Bus Isolation](https://taphome.com/en/docs/wiring/optical-isolation/), [Wiring](https://taphome.com/en/docs/wiring/).