A thread about a trap some guy made

[Flashblade]

This is the thread, I'm the guy, and here's the trap:

Electric Conduit (CR 7)

Every round, 3 electricity discharges arc across the length of this 20-ft wide conduit. For each electricity discharge, roll 1d4 to determine which 5-ft. wide channel is struck. The entire trap may be disabled at a central location halfway up the conduit, and the individual channels may be disarmed at either end of the conduit.

XP 3,200
Type mechanical; Perception DC 15; Disable Device DC 25
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 EFFECTS
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Trigger location; Reset automatic; Bypass hidden disarm switch (Perception DC 25, Disable Device DC 30)
Effect 3 electricity discharges (3d6 electricity damage, DC 20 Reflex for half damage); multiple targets (all targets in a 120-ft. line)

CR Maths:
• Mechanical, +0 CR
• Perception 15, –1 CR
• Disable Device 25, +1 CR
• Reflex DC 20, +0 CR
• Average damage 31.5 (multiple targets, ×2), CR +6
• Automatic reset, CR +1

Do you folks think this is balanced for the CR? Too easy or hard? Is the description ambiguous? How could the write-up be improved?

[Abraham spalding]

Seems right to me honestly.

[Dave Justus]

Your average damage for any individual channel is 7.5, not 31.5. Unless an individual is large, they will only be in one at a time. That would make the CR Only 2. I would probably bump it up a bit, since the variable channel adds to how much this damage can 'swing' but I don't think it is worth being a CR7, 4 or 5 seems more like it, especially if it is pretty obvious how it will work.

[Kaisoku]

Flashblade wrote:

Every round, 3 electricity discharges arc across the length of this 20-ft wide conduit. For each electricity discharge, roll 1d4 to determine which 5-ft. wide channel is struck.

So since it's random, a person might not get hit at all, or may get hit with all three (if the d4s roll the same every time).

However, with 4 channels and only 3 charges, there's quite a bit of leeway.

If you apply the odds of being in the channel against the damage of being hit with 1, 2 or 3 charges, you get the following:

3d4 chance to get hit with the first charge: 75% x 10.5 = 7.875
The odds of being hit a second time would be 3/4 * 3/4, or 75% * 75% x 10.5 = 5.90625
The third hit would be 75% * 75% * 75% x 10.5 = 4.4296875

Add all three hits together: ~18.21

Rounding brings us to 20 damage, and multiple targets brings that to 40. So a +4 CR for damage. Overall a CR 5 trap (as Dave Justus suggested, actually).

Edit: Hmm... I'm second guessing myself on the probabilities there. It's been nearly two decades since I did any serious statistics... perhaps there's someone else out there that might know better.

[Pizza Lord]

Not sure it really counts as a trap unless your electricity arcs are not visible. That's like saying a power line is a trap or an electrical socket is a trap if you stick a fork in it (as opposed to an electrical socket that looks like a lock or heyhole).

To be considered a trap it would either have to be difficult to detect the danger to a normal person looking at it (and this isn't, since you state 'arcs of electricity) or it needs a trigger, such as 'a living creature in the conduit) or something similar.

Perception check probably isn't really necessary (It would be to find the secret compartment containing the bypass). It's about as much a trap as an open pit in the ground that is clearly visible.

Otherwise, like Dave Justus and Kaisoku mention, the actual damage potential is probably just CR 2 range (unless you add more danger, like anyone getting shocked must make a balance check or a Fortitude check to avoid muscle convulsions and falling off the conduit, or to avoid being momentarily stunned and knocked prone for a round, thus giving the potential of more shocks.)