I/O surge protection module: DIN-rail signal-line SPD for PLC inputs and 4–20 mA loops

01

Uc above the loop voltage, then In and Up for the IEC 61643-21 test category

Pepperl+Fuchs' M-LB-4252, a 6 mm surge protection barrier for two non-grounded signal lines, is rated for a nominal voltage of 48 V and a maximum continuous operating voltage Uc of 56 V DC (39.6 V AC). It diverts a nominal discharge current In of 5 kA per line (8/20 µs), a total of 10 kA, and 1.5 kA per line of 10/350 µs impulse current, with a maximum surge current of 20 kA; the voltage protection level Up at In is at most 90 V line-line and 600 V line-earth. [1]

DEHN's BLITZDUCTOR XT module BXT ML2 BE S 24 is rated for 24 V with Uc of 33 V DC (23.3 V AC). It lists 10 kA per line and 20 kA total at test category C2 (8/20 µs), 2.5 kA per line and 9 kA total at D1 (10/350 µs), and voltage protection levels of at most 102 V line-line and 66 V line to earth at D1, and 90 V and 45 V at C3 (1 kV/µs). [2]

The letters come from the test classes of EN 61643-21: ABB's Furse reference describes class D as high energy, C as fast rate of rise and B as slow rate of rise for SPDs on data, signal and telecom lines, and notes that the companion specification 61643-22 covers selection and installation. [3] Editorial note: compare Up only at the same test category; DEHN's 45 V at C3 and Pepperl+Fuchs' 600 V line-earth at In are not the same test.

Editorial calculation: a 24 V loop on the DEHN module leaves 33 − 24 = 9 V between nominal and Uc, while the 48 V-rated Pepperl+Fuchs module leaves 56 − 24 = 32 V on the same loop; the trade-off is the higher clamp level of the higher-voltage part. [2] [1]

Give the supplier the signal type (24 V digital, 4–20 mA, RS-485 or other), the loop voltage and maximum current, whether the lines are floating or referenced to earth, the I/O card's withstand voltage, the cable route (between buildings, outdoors, inside one cabinet), any SIL or Ex zone requirement, and the earthing arrangement in the cabinet.

02

1 Ω to 1.8 Ω per line in the 4–20 mA loop, and leakage below 30 µA

Each module puts resistance in series with the signal: 1 Ω per line on the Pepperl+Fuchs M-LB-4252 and 1.8 Ω per line on the DEHN BXT ML2 BE S 24. Both list a rated load current of 0.75 A, at 70 °C for Pepperl+Fuchs and at 45 °C for DEHN. [1] [2]

Editorial calculation: a 2-wire loop through the DEHN module passes both lines through 1.8 Ω, so 3.6 Ω at 20 mA drops 0.072 V; a second module at the field end doubles that to 0.144 V. The loop supply must cover this on top of the transmitter's minimum voltage and the input resistance of the I/O card. [2]

Pepperl+Fuchs lists leakage below 30 µA line-line at Uc and below 10 nA line to earth. Editorial calculation: 30 µA is 0.19% of the 16 mA span of a 4–20 mA signal; since the figure is stated at Uc (56 V DC), ask for the leakage at your loop voltage before treating it as an error budget item. [1]

Bandwidth limits apply to faster signals: Pepperl+Fuchs gives a cut-off frequency of 3.6 MHz line-line, DEHN 6.8 MHz line to earth with capacitance of at most 0.5 nF line-line and 1.0 nF line to earth. [1] [2]

03

Status display, plug-out protection modules and earthing through the base

Both modules split into a base wired to the field and a plug-in protection part. Pepperl+Fuchs' protection module can be replaced without tools, the device resets itself after an impulse in under 30 ms, and a front display shows operating state in green and fault in red. [1]

DEHN's module plugs into a BXT BAS or BSP BAS 4 base part and earths through it. Its LifeCheck function detects thermal or electrical overload; the status is read contactlessly with a DEHNrecord reader, and an overloaded arrester has to be replaced. [2]

For safety loops and hazardous areas, both datasheets list use up to SIL 3 and Zone 2 markings: Ex ec IIC T4 Gc for Pepperl+Fuchs (TÜV 22 ATEX 8881 X) and Ex nA IIC T4 Gc for DEHN (DEKRA 11ATEX0089 X). [1] [2]

Editorial note: these modules divert surge current to earth, so the clamp level reached at the I/O card depends on the connection from the base to the cabinet earth as well as on the module; ask the supplier for the installation instructions and keep that path short.

04

Two signal-line SPD modules for 24–48 V I/O side by side

05

Send the signal, the cable route and the earthing together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Signal: [24 V digital; 4–20 mA; RS-485; other]
Voltage: [loop or supply voltage; maximum current]
Lines: [number per channel; floating or earth-referenced]
Surge level: [test category and current per line required]
Loop budget: [series resistance allowed; leakage allowed]
Safety and Ex: [SIL; zone and marking]
Mounting: [module width; status indication; pluggable]
Earthing: [cabinet earth bar; base-to-earth connection]

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