Allen Bradley 800T/H 30mm Button Troubleshooting and Replacement Guide

Contact block burnout or contact adhesion due to excessive load (exceeding 10A resistive/A600) or frequent starting of large motors to check the actual load; Replace with a higher capacity contact block or use an external contactor

2.2 Essential Tools and Safety Preparation

Digital multimeter (with on/off buzzer mode)

Small flathead screwdriver (used for pressing self elevating terminals)

Replacement contact block (standard, PentUFF or explosion-proof type)

Replace with button head (same color and operation type)

Spare sealing gasket

Safety measures: Before conducting live inspections, be sure to disconnect the relevant control circuit power supply and hang a lock and tag. When testing the continuity of contacts, use resistance mode and do not apply external voltage.

2.3 Deep diagnosis of common faults in contact blocks

There are three mainstream types of 800T/H series contact blocks, each with different fault symptoms:

a) Standard contact blocks (800T-A, 800T-B, etc.)

Features: 10A max, A600 rated, pure silver contacts, bifurcated contact bridge.

Common faults: Silver contacts sulfide and turn black (in sulfur-containing environments), causing contact resistance to rise to several ohms and PLC input detection failure.

Solution: Gently wipe the contact surface with fine sandpaper (# 600), do not file. When severe oxidation occurs, the contact block should be replaced.

b) PentUFF ™ Contact block (low-level switching)

Features: 2.5A max, C300 rated, pentaforked design, using inert precious metals (palladium/gold), reliable operation at 5V DC, 1mA signal - specifically designed for PLC input circuits.

Common faults: Although not easily oxidized, mechanical impact may cause changes in contact gap. If the PLC input still shakes, check if the spring plates between the contacts are deformed.

Solution: This type of contact block is not suitable for directly switching inductive loads (such as contactor coils) and must be used for PLC input or solid-state loads. If a large load is mistakenly connected, the contacts will melt.

c) Explosion proof contact block (Explosion Protected)

Features: Certified Class I Div. 2 (Groups A, B, C, D), A600 rated, normally closed contacts with positive opening, in compliance with global emergency stop standards.

Common faults: In a vibrating environment, screw type terminal blocks may become loose, causing intermittent disconnections.

Solution: Regularly use a torque screwdriver to tighten the terminals according to specifications (recommended 0.8 N · m). Check if the positive opening mechanism is stuck by foreign objects.

Chapter 3: Steps for Replacing and Upgrading Contact Blocks

3.1 Factory pre installed contact block button (no on-site assembly required)

Most buttons in the 800T/H series are pre installed with contact blocks according to the ordering code when they leave the factory. For example:

800T-A1D1=metal button, green, instant action, 1NO.

800T-A2A=metal button, red, instant action, 1NO+1NC.

If the contact block is found to be damaged on site, the button assembly can be disassembled for replacement without removing the entire button head.

Disassembly and replacement steps:

Power off, mark and remove the wires.

Disassemble the button head: Hold the mounting ring with one hand and rotate the button head counterclockwise with the other hand, usually about 1/4 turn to remove it.

Remove the contact block module: The contact block is fixed to the rear of the operator by a buckle or screw. Gently pry open the buckle (or loosen the screw) and slide the contact block out.

Install new contact block: Ensure that the locating pin is aligned and pushed in/tightened. Note that the NO/NC configuration of the contact block should be consistent with the original.

Check the diaphragm seal: If the diaphragm is damaged or hardened, the entire button head must be replaced (the seal cannot be replaced separately).

Reinstall: Rotate the button head clockwise to install the ring, and lock it when you hear a "click" sound. The installation torque is about 1.1 N · m (excessive tightening can damage the seal).

Conduct test: Before restoring power supply, use a multimeter to confirm that the contact continuity is consistent with the drawing.

3.2 Precautions for Upgrading to PentUFF Contact Blocks

When the control circuit is upgraded from a hard wired relay to a PLC input, the original standard contact blocks (even if the contacts are not oxidized) may malfunction due to minimum load requirements (usually 5V/10mA). The solution is to replace it with a PentUFF contact block.

Compatibility: The size of the PentUFF contact block is exactly the same as the standard contact block and can be interchanged. However, it should be noted that:

The PentUFF model starts with "Q", for example, 800T-QH2R (with light, universal LED, 1NO+1NC).

When placing an order, it is necessary to select the correct contact block configuration, such as changing from 800T-A1A to 800T-Q2A (to be verified). In actual replacement, it is recommended to purchase the entire button head assembly with PentUFF contact blocks, or separately purchase contact block kits such as 800T-QB2A.