Terminal blocks are where drawings go vague and assembly goes wrong. The RFQ needs the wire cross-section, the voltage, the current, the clamp, and the extras: jumper, test point, fuse, or disconnect. A photo of a grey block is not a specification.
The lines that change the part number
Wire range is a minimum and a maximum. A block rated 0.5 to 2.5 mm2 will not hold a 6 mm2 power wire, and it may not hold a single fine strand without a ferrule. Say whether ferrules are required. Current rating is per pole at an ambient. If the rail is full and the cabinet is 55 C, ask for the derating curve rather than the 20 C headline.
Pitch and height decide whether the block fits the duct and the door. Multi-level blocks save rail and complicate the jumper and the mark. Disconnect and fuse blocks cost more because they add a function. Call that function out. A feed-through block cannot be swapped for a knife disconnect after the panel is punched.
- Wire size, solid or stranded, and ferrule if you use one.
- Voltage and current, and the ambient if it is above 40 C.
- Clamp: screw, spring, or push-in, and the tool the line already has.
- Pitch, levels, and color if your standard fixes earth and neutral colors.
- Accessories: end cover, jumper, marker, test plug, shield connection.
A minimum table for the drawing
| Pole duty | Call out | Typical miss |
|---|---|---|
| Power | Cross-section, current, voltage | Buying the signal pitch |
| Signal | Wire range and whether it is shielded | No shield terminal |
| Earth | A block rated as protective earth | Painted rail used as the only path |
| Fused | Fuse size and indicator if you need one | A feed-through substituted later |
Check a sample before the reel arrives
Land the largest and the smallest wire you specified. Torque a screw clamp to the datasheet value, or close a spring clamp and tug. Confirm the marker can be read from the aisle. Confirm the jumper actually fits that series. Series look alike and the jumper does not cross families. Keep the datasheet page that states voltage, current, and wire range with the approved sample.
What to lock on the order
Number the terminals on the drawing and use the same numbers on the marker strips. The RFQ should say how many of each type, not a single line that says terminal blocks, assorted. Group power, signal, and earth as separate lines with their own wire range. State the rail type, almost always the common DIN rail, and whether the blocks must be the same series so one jumper system works. A mixed series looks tidy and then the jumper will not cross. Ask the assembler which screwdriver or which ferrule tool the line already owns, and prefer the clamp that tool can build consistently. On the sample, mark one block and photograph the wire range printed on the side. That photograph is the approval, next to the datasheet page.
Questions buyers actually ask
Are spring and screw terminals equal?
They both can be listed for the same current. The difference is vibration, rework, and whether the assembler has a torque tool. Write the clamp style you need.
What pitch should I call out?
The pitch has to fit the wire ferrule and the adjacent block. A smaller pitch saves rail and rejects a large ferrule.
Do I need a ground block?
If a protective earth lands on the rail, use a block the maker rates for that, in the color your standard requires. Do not rely on the metal foot alone unless the datasheet says the foot is the earth path.