The CMC MPD, and the argument it started
CMC's Multi-Purpose Device collapsed a pulley, a belay and a descent control device into one unit — and in doing so helped change how rescue teams rig. Not everyone thinks that change was an improvement.
Most gear gets described by what it replaces. The MPD is unusual because what it replaced wasn’t a product — it was a habit.
CMC has been building rescue equipment out of Santa Barbara since 1978, and the MPD is the thing the company is best known for. The MPD is a pulley, a descent control device and a belay, in one housing, on one carabiner. CMC says more than nine years of development went into it, and it’s patented.
That’s the sales pitch, and it’s accurate. The more interesting story is what happened to rigging practice afterward.
What’s actually in the housing
The MPD is a high-efficiency sheave with an integral rope-locking cam. Pull rope through one way and it’s a pulley in a haul system, with the cam acting as progress capture. Release the handle and it’s a descent control device that lowers under control. Stop, and it holds.
The number that matters most in the field is that you can go from lowering to raising without changing hardware. On a real edge, with a litter and an attendant hanging off it, not having to break the system down and rebuild it in the other direction is worth more than any single specification.
Per CMC’s own manual, both models weigh 1.1 kg (2 lb 8 oz). Pulley minimum breaking strength is 46 kN (10,341 lbf); the becket is rated 26 kN (5,845 lbf). The current units are UL certified to NFPA 2500 (2022 edition) — the consolidated standard we wrote about last time.
The spec split most people never read
There are two MPDs, and they are not the same device with different paint.
The 13 mm (1/2 in) model, part 333000, is rated General Use across all three functions — pulley, descent control and belay.
The 11 mm (7/16 in) model, part 333010, is rated General Use as a pulley, but only Technical Use as a descent control device and as a belay.
That distinction is the difference between a device certified for two-person loads and one certified for one. If your team has standardized on 11 mm rope — and plenty have, for weight and packability — you have quietly bought a Technical Use lowering system. That may be entirely correct for your operations. It’s a bad thing to discover during an audit, or worse, while working out whether the system in front of you is rated for the litter you’re about to put on it.
Nothing about this is hidden. It’s printed in the manual and stamped on the device. It’s just that “we run MPDs” is the sentence people say, and the sentence doesn’t carry the rating.
What it changed
Traditional rope rescue ran a dedicated main line and a dedicated belay. The main carries the load. The belay hangs slack, doing nothing, until the main fails — at which point it catches everything at once. That shock load is the whole problem with the model: a belay catching a failed main can see forces beyond 12 kN.
Two-tensioned rope systems — TTRS, or dual-capability two-tensioned systems — put both ropes under tension and share the load between them. If one fails, the other is already loaded and already taking its share, so the event is a load transfer rather than a catch. Reported forces stay in roughly the 6–12 kN band instead of spiking.
You can’t practically run that system with a pulley on one rope and a belay device on the other. You need two devices that behave identically, lower identically, hold identically and convert to haul identically. Two MPDs do exactly that.
So the device didn’t just save space in a kit bag. It made a rigging philosophy practical, and that philosophy spread.
The part a product page won’t tell you
Two-tension systems are not universally accepted, and the pushback comes from people worth listening to.
Rigging for Rescue’s Gibbs and Koprek published a detailed critique arguing the practical difference between two-tension and traditional main-and-belay comes down to roughly four meters of edge transition — after which both systems converge and behave much the same. Their specific findings are worth sitting with:
On sharp edges, the advantage showed up only in extreme conditions — unprotected, exceptionally sharp edges of the kind rarely met in real rescues. With adequate edge protection in place, they found no difference in performance between the two approaches. Every rope survived.
On stopping distance during that initial transition, the elongation difference between systems was centimeters, not meters.
And on the assumption underneath the whole model — that both ropes share the load equally — they note that across more than 120 training seminars since 2016, they rarely saw practitioners actually achieve equal tension. A system whose safety case rests on 50/50 sharing, in the hands of people who mostly don’t achieve 50/50, is a different proposition than the one on paper.
Their sharpest point is about opportunity cost. Running two-tension well takes real training time. Incident data, they argue, shows that poor inspection, poor rigging, losing control of the running end, and bad command and communication account for far more harm than rope trauma on an edge. Training hours spent chasing the four-meter problem are hours not spent on the failures that actually recur.
None of this makes the MPD a bad device. It’s an argument about method, and the MPD is equally capable of running a traditional main-and-belay. But it’s a useful corrective to the idea that buying the hardware buys the outcome.
Who should own one
If you run technical rescue and you’re building or replacing a lowering system, the MPD is the reference point everything else gets measured against. The lowering-to-raising conversion alone justifies it for litter work, and the build quality is what you’d expect from a company that has only ever made rescue equipment.
If you’re a rope access technician working two-rope SPRAT or IRATA systems, this is probably not your device. It’s heavy for personal kit and it solves a team-rigging problem you don’t have.
And if you’re buying a pair to run two-tension: budget the training with the hardware, not after it. On the evidence, that’s where the actual safety lives.
Full write-up on the manufacturer is on the CMC brand page. We use and recommend CMC; nobody paid for this, and when they become a stockist we’ll say so on the page.