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Aha! I knew I'd read something about this somewhere, and here it is. Scroll down to the test done by Jim Titt.

First Fail Mode:

The trapped rope escapes sideways from under the tensioned rope and gets trapped between the tensioned rope and the side of the slot.This is very difficult to free off and you have to dismantle everything and twist the locking krab brutally to release the rope. Take your Prusiks.

Second fail mode:

Apply yet more load and the trapped rope where it crosses the tensioned rope goes down through the slot with a bang. At this point the holding power drops off considerably but not catastrophically, though pretty near! Easy to release, just unclip the krab when unweighted. Still need to take your Prusiks!

ATC Guide. 10.2 Mammut, used, non-treated. First fail mode 4.8kN. No second fail mode, rope sheath cut at ca 9kN.

ATC Guide. 9mm Edelrid, used, non treated. First fail mode 2.96kN. Max fail load 5.58kN. Residual load 1.6kN

ATC Guide. 8.2mm Edelrid, new,treated. First fail mode 2.05kN. Max fail load 4.06kN. Residual load 1.2kN

Reverso�. 10.2 Mammut, used, non treated. First fail mode 3.68kN. No second fail mode. Rope sheath cut ca 9kN

Reverso�. 9mm Edelrid, used, non treated. First fail mode 2.25kN. Max fail load 3.60kN. Residual load 0.9kN

Reverso�. 8.2mm Edelrid, new,treated. First fail mode 1.6kN. Max fail load 2.38kN. Residual load 0.7kN All with Petzl Attache 12mm round profile karabiner.

Not my idea of a reliable roped-solo device!

https://web.archive.org/web/20150630222503/http://www.mountainproject.com/v/atc-guide/106838345__1

He pull tested an ATC in guide mode. Quick summary: the device itself doesn't fail or suffer damage. With a 10mm rope the ATC jams severely at 4.8 kN, breaks the rope at 9 kN. I don't know how to work out how much of a fall the second would have to take to get to 4.8 kN, but probably quite a bit; s/he certainly ain't getting to nine.

With an 8mm rope it's a little more alarming; device jams at 2 kN, locking fails at 4 kN. I imagine 2 kN would be pretty easy for a dropped follower to achieve, so anybody belaying two followers simultaneously on skinny double ropes evidently needs to pay attention.

Aha! I knew I'd read something about this somewhere, and here it is. Scroll down to the test done by Jim Titt.

http://www.mountainproject.com/v/atc-guide/106838345__1

He pull tested an ATC in guide mode. Quick summary: the device itself doesn't fail or suffer damage. With a 10mm rope the ATC jams severely at 4.8 kN, breaks the rope at 9 kN. I don't know how to work out how much of a fall the second would have to take to get to 4.8 kN, but probably quite a bit; s/he certainly ain't getting to nine.

With an 8mm rope it's a little more alarming; device jams at 2 kN, locking fails at 4 kN. I imagine 2 kN would be pretty easy for a dropped follower to achieve, so anybody belaying two followers simultaneously on skinny double ropes evidently needs to pay attention.

Aha! I knew I'd read something about this somewhere, and here it is. Scroll down to the test done by Jim Titt.

First Fail Mode:

The trapped rope escapes sideways from under the tensioned rope and gets trapped between the tensioned rope and the side of the slot.This is very difficult to free off and you have to dismantle everything and twist the locking krab brutally to release the rope. Take your Prusiks.

Second fail mode:

Apply yet more load and the trapped rope where it crosses the tensioned rope goes down through the slot with a bang. At this point the holding power drops off considerably but not catastrophically, though pretty near! Easy to release, just unclip the krab when unweighted. Still need to take your Prusiks!

ATC Guide. 10.2 Mammut, used, non-treated. First fail mode 4.8kN. No second fail mode, rope sheath cut at ca 9kN.

ATC Guide. 9mm Edelrid, used, non treated. First fail mode 2.96kN. Max fail load 5.58kN. Residual load 1.6kN

ATC Guide. 8.2mm Edelrid, new,treated. First fail mode 2.05kN. Max fail load 4.06kN. Residual load 1.2kN

Reverso�. 10.2 Mammut, used, non treated. First fail mode 3.68kN. No second fail mode. Rope sheath cut ca 9kN

Reverso�. 9mm Edelrid, used, non treated. First fail mode 2.25kN. Max fail load 3.60kN. Residual load 0.9kN

Reverso�. 8.2mm Edelrid, new,treated. First fail mode 1.6kN. Max fail load 2.38kN. Residual load 0.7kN All with Petzl Attache 12mm round profile karabiner.

Not my idea of a reliable roped-solo device!

https://web.archive.org/web/20150630222503/http://www.mountainproject.com/v/atc-guide/106838345__1

He pull tested an ATC in guide mode. Quick summary: the device itself doesn't fail or suffer damage. With a 10mm rope the ATC jams severely at 4.8 kN, breaks the rope at 9 kN. I don't know how to work out how much of a fall the second would have to take to get to 4.8 kN, but probably quite a bit; s/he certainly ain't getting to nine.

With an 8mm rope it's a little more alarming; device jams at 2 kN, locking fails at 4 kN. I imagine 2 kN would be pretty easy for a dropped follower to achieve, so anybody belaying two followers simultaneously on skinny double ropes evidently needs to pay attention.

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Aha! I knew I'd read something about this somewhere, and here it is. Scroll down to the test done by Jim Titt.

http://www.mountainproject.com/v/atc-guide/106838345__1

He pull tested an ATC in guide mode. Quick summary: the device itself doesn't fail or suffer damage. With a 10mm rope the ATC jams severely at 4.8 kN, breaks the rope at 9 kN. I don't know how to work out how much of a fall the second would have to take to get to 4.8 kN, but probably quite a bit; s/he certainly ain't getting to nine.

With an 8mm rope it's a little more alarming; device jams at 2 kN, locking fails at 4 kN. I imagine 2 kN would be pretty easy for a dropped follower to achieve, so anybody belaying two followers simultaneously on skinny double ropes evidently needs to pay attention.