Slim Body or Compact Block? Choosing Contact Configuration and Format for Guard Locking Switches

Two questions decide which guard locking interlock lands on your bill of materials, and they are usually asked in the wrong order.

Most buyers start with contacts and discover at installation that the switch does not fit the frame. Start with the frame, then work out the contacts — and check both before the PO goes out.

Part of our guard locking series: buyer's guide  •  spring vs solenoid lock  •  IS 16812 / ISO 14119  •  wiring guide  •  price guide  •  power press safety  •  actuator keys  •  defeat prevention.

Safety Interlock Switches In Stock

Genuine guard locking and tongue-actuated safety interlock switches from Allen-Bradley, Schmersal, Omron, Telemecanique and Banner Engineering — in stock for dispatch across India with GST invoice.

Browse the full range on our brand pages: Allen-Bradley, Schmersal, Omron, Telemecanique and Banner Engineering.

Question 1: Will it physically fit?

Guard locking interlocks come in two dominant formats.

Slim tall body (approx. 39 × 39 × 183 mm)

Long and narrow, with mounting holes in two pairs down the length. Use it when:

• Mounting on a fence post or square-section frame member.
• You have vertical run but very little width.
• You need cable entries on multiple faces — three M20 entries (left, right and bottom) let you route cables however the frame demands.
• The safety design needs door and lock circuits kept independent.

Watch out for: the length. On shallow guard frames and short sliding door stiles there is simply nowhere to put 183 mm of switch body.

Compact block (approx. 108 × 89 × 36 mm)

Roughly square and shallow, mounted on four screws. Use it when:

• The frame is shallow and the slim body will not fit.
• You are working on a sliding door with a short leading edge.
• Body length is restricted but you have surface area.
• You want more contacts in a smaller footprint.

Watch out for: typically a single M20 entry at the base, so cable routing is fixed. Plan the gland direction at design stage.

Measure the frame member — width, depth and available run — before selecting. This one check prevents most of the “it arrived and it doesn't fit” problems in this product category.

Question 2: How many contacts, and how are they linked?

This is where the safety architecture is decided.

Four contacts: 1NC + 1NO door, 1NC + 1NO lock

The volume configuration. You get:

• One NC safety contact following the actuator (door closed).
• One NC safety contact following the lock bolt (bolt extended).
• Two NO contacts for PLC status — “guard open” and “unlock confirmed”.

Wire the two NC contacts in series into a safety relay and the machine can only run when the guard is both closed and locked. That is single-channel safety with full status feedback — correct and sufficient for a great many machines.

Because the door and lock circuits are independent, you can also evaluate them separately where the safety design requires the control system to distinguish “closed” from “locked” on separate channels.

Six contacts: 2NC + 1NO door, 2NC + 1NO lock

Three contacts per block. The critical detail is how the manufacturer links them.

On devices where the blocks are cross-linked internally — typically terminal 12 to 41 and 22 to 51 — each pair forms one complete series “door closed and locked” NC channel. You end up with two such channels available at, for example, terminals 11-42 and 21-52.

Feed those into a dual-channel safety relay with cross-fault detection and you have Category 3 / 4 architecture from a single device, plus two independent NO contacts still free for PLC status.

The trade-off: because the cross-links are made inside the switch, the door and lock functions can no longer be evaluated as separate safety channels. If your design needs independent door and lock circuits, use the four-contact independent format instead.

Side-by-side

The contact code, decoded

Model numbers in this category follow a consistent logic once you know it:

• NC = normally closed, NO = normally open.
• A digit prefix gives the quantity: 3NC is three normally closed, 2NC1NO is two normally closed plus one normally open.
• D- denotes the door (actuator) monitoring block.
• L- denotes the lock (bolt) monitoring block.

So D-2NC1NO-L-2NC1NO reads as: two NC plus one NO on the door block, two NC plus one NO on the lock block — six contacts in total.

Some series offer a wide catalogue of combinations — door-priority variants with three NC on the door block for separate isolation circuits, lock-priority variants with multiple channels following the bolt, and configurations weighted towards auxiliary status outputs. If your standard configuration is not on the shelf, ask: the variant probably exists.

Both formats in stock

The Voxintech VXT-SS series covers both:

• VXT-SS-W2-D-NCNO-L-NCNO — slim tall body, 39 × 39.4 × 183 mm, 4 contacts, three M20 entries, independent door and lock circuits. Fourteen contact combinations available across the series.
• VXT-SS-W5-D-2NC1NO-L-2NC1NO — compact block, 108 × 89 × 35.5 mm, 6 contacts, internally cross-linked to give two complete series safety channels at 11-42 and 21-52.

Both share 1300 N locking force, 24 V DC solenoid, IP67, metal rotatable head, forced-disengagement contacts and the same eleven interchangeable actuator keys — so you can mix formats across a machine without doubling your actuator stock.

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Frequently Asked Questions