Where are lockers required?

Seems to me there is a general (but not universal!) consensus on applications where locking biners are required.  Belay biners, when used in anchor construction, off a masterpoint when TRing, when rappelling, when clipping in to multi-pitch belay stations.  I'm wondering how these were developed, especially when compared to other situations such as quickdraws, where relying on a single non-locker is common and rope interaction is likely.  I'm assuming these have just been developed over time balance safety, convenience, and risk, but is there any other logic/science to it?  

Some use lockers on quickdraws where an open gate or possible unclipping could be fatal, like the first good piece after a long runout with another run out above.
Risk management.

Any single point of failure. Two opposite and opposed non locking crabs are just as good or perhaps even better than a single locking crab.

Like all insurance, you only need it when you need it.

You'll never get agreement on an answer because no one can be certain until after the fact. If a biner opens and someone dies, a locker was required. Otherwise it's all speculation.

Cam Hook wrote: Any single point of failure. Two opposite and opposed non locking crabs are just as good or perhaps even better than a single locking crab.

Define single point of failure.

Jason Kim wrote:

Define single point of failure.

Your belay loop is an example. 

In all seriousness, a single point a failure is where a carabiner failure would have catastrophic consequences. A single point of failure would be a place in the system where there is no redundancy in the system. Belay carabiner, Master point carabiner, etc. Two non lockers on a two bolt anchor is fine as the anchor has redundancy. Pretty simple to answer but the answer might depend on the context. 

Jason Kim wrote:

Define single point of failure.

A 3 piece anchor going to 1 attachment point on your harness (ie belay loop) 

Cam Hook wrote:

Your belay loop is an example. 

In all seriousness, a single point a failure is where a carabiner failure would have catastrophic consequences. A single point of failure would be a place in the system where there is no redundancy in the system. Belay carabiner, Master point carabiner, etc. Two non lockers on a two bolt anchor is fine as the anchor has redundancy. Pretty simple to answer but the answer might depend on the context. 

This is the answer for the real world.

There are a lot of climbers that have way more lockers than they need.

-Insert obnoxious comment here:_______________________

YGD

Lockers aren't necessarily required everywhere. There's no omnipresent climbing god who's going to smite you if you don't use a locker on your belay device, though mountain project forums might come close. People climbed for years without lockers, they just carried like 1500 carabiners up a route.

But lockers make ensuring security on that single point of failure a whole lot simpler. For example, you could use two non-lockers on a belay device, and in fact some people still do. You could use two non-lockers to connect yourself to an anchor, and in fact, some people do, myself included on occasion. But a single locker in both those situations is usually simpler than having two non-lockers. While having two carabiners opposite and opposed might be more secure, there are some cases where you don't need the security of two carabiners, but you want more than the security of a single non-locker. That's where you probably want to use a locker.

Are the results of carabiner failure catastrophic?

No- No locker

Yes- Locker or opp/opposed non lockers

Yes, *and* we're leaving it unattended/can't get to it (as in TR masterpoint) - Two lockers opp/opposed

high school gyms

Any place where a carabiner coming unclipped is going to catastrophic. This would be failure points like your belay carabiner or attaching yourself into the anchor. Toprope anchors don't need to be all lockers but they won't hurt. I would rather have 2 opposite and opposed biners on a TR anchor than a single locker where you're belaying from the ground as lockers can still become unlocked and since they're unattended are more likely to misbehave.

Cam Hook wrote:

Your belay loop is an example. 

In all seriousness, a single point a failure is where a carabiner failure would have catastrophic consequences. A single point of failure would be a place in the system where there is no redundancy in the system. Belay carabiner, Master point carabiner, etc. Two non lockers on a two bolt anchor is fine as the anchor has redundancy. Pretty simple to answer but the answer might depend on the context. 

Agreed! Just figured it would be good to clarify for beginners.  I used way too many lockers early on, until it dawned on me that it was fairly pointless to carry the extra weight.

Much of this is already covered. Still, for me, the shortest general answer of where to use a locker is when

1. failure would result in significant harm or death.
2. and an open gate could cause the connection to fail 
3. and a non-locking gate could reasonably be pushed or knocked open.

Edit: Failure modes include an open biner  breaking because it is not as strong as one with a closed gate (assuming large enough force); or a sling or bolt hanger that slips out of an open gate biner. But this is not an all inclusive list.

Don’t forget that a locker only insures that the gate will stay closed. They are not any stronger than any other biner. 

Mark Hudon wrote: Don’t forget that a locker only insures that the gate will stay closed. They are not any stronger than any other biner. 

That may be nearly true in optimal loading situations, but if there may be sideways/twisting loading, I’d rather have a Rocklock there than a wafer thin Nano.

When said carabiners failure will result in catastrophe.

Dan Gozdz wrote: Any place where a carabiner coming unclipped is going to catastrophic. This would be failure points like your belay carabiner or attaching yourself into the anchor. Toprope anchors don't need to be all lockers but they won't hurt. I would rather have 2 opposite and opposed biners on a TR anchor than a single locker where you're belaying from the ground as lockers can still become unlocked and since they're unattended are more likely to misbehave.

Also, 2 opposite and opposed carabiners have the advantage of not getting stuck. Over the years, many reports of  a climber being attached to the anchor using a sling and a locking carabiner and the locker being stuck in the closed position. If the climber doesn't have a knife to cut the sling, then the climber is stuck. 

Yes I read the thread.
The answer in one word is "redundancy"

Sloppy Second wrote:

This is the answer for the real world.

There are a lot of climbers that have way more lockers than they need.

Some use them just for peace of mind.

I once scored 4 lockers someone left on a 2-bolt sport anchor.