Product Overview
NEMA A600 Open Frame DC Switching Relay with Convertible Contacts
Condition
Unused new old stock (NOS). Terminals, contact surfaces, and zinc-plated baseplate show no signs of installation or arc wear. Markings legible: Square D Class 8501 Type HDO 40, Series B; coil stamped 31071-412-23 SDJ 24VDC. Sold each.
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Product Overview
Square D Class 8501 Type HDO-40 is a heavy-duty industrial DC control relay designed for open-frame panel installation. Built with four convertible contact poles and an integrated coil economizer resistor assembly, it provides reliable switching logic in industrial DC control schemes and motor starters. Each contact pole converts between normally open (N.O.) and normally closed (N.C.) operation by inverting the contact assembly and lower terminal pointer.
Key Features
- Coil operating voltage: 24 VDC
- Coil module identification: 31071-412-23 SDJ with factory resistor housing assembly
- Contact poles: 4 convertible poles (reversible N.O. or N.C.) plus dedicated overlapping contact for resistor circuit
- AC contact rating: NEMA A600 (600 VAC maximum)
- DC contact ratings: 1.1 A at 120 VDC, 0.55 A at 240 VDC, 0.20 A at 600 VDC maximum make or break
- Terminal style: Screw terminals with captive clamping plates
- Mounting style: Steel baseplate with slotted mounting holes for panel installation
- Construction: Open-frame industrial relay, Series B
Applications
- Replacing worn or pitted DC control relays in existing machine tool panels without altering panel layout.
- Rebuilding legacy equipment control cabinets that depend on 24VDC control coils and NEMA A600 contact ratings.
- Configuring custom DC sequencing circuits for workshop automation, shop presses, or winch controllers where contacts need quick field conversion between N.O. and N.C.
Frequently Asked Questions
How are the contact poles converted between normally open and normally closed?
Each pole converts individually without extra parts. Loosen the captive lower terminal mounting screw, rotate the contact assembly until the desired legend (N.O. or N.C.) is visible and seated, invert the lower terminal so the pointer matches the legend, and retighten the screw.
What is the purpose of the resistor housing assembly on this relay?
The resistor assembly and overlapping contact form a coil economizer circuit for the 24VDC coil. It delivers full pick-up voltage to pull in the armature, then switches the series resistor into the coil circuit to drop continuous holding current and reduce operating heat.
Can the contacts switch AC loads even though the coil operates on 24 VDC?
Yes. The contacts are rated NEMA A600 for up to 600 VAC switching, while the operating coil and resistor assembly require 24 VDC to energize.
Before logic chips took over cabinet space, heavy-duty iron like the 8501 did the thinking with a satisfying mechanical clatter. Bolt this relay into your panel, wire the coil, and let proven American switchgear run the cycle.
