I have always laughted at and thought how dumb do the writers think I am.
Sure bombers follow the IBEC. If they don’t their bomb won’t pass inspection. And if it does not pass inspection then they can not use the bomb.
By the way the IBEC is the International Bomber’s Electrical Code.
Aren’t the wires just competing a circuit? Why does it matter which one you cut?
It matters because sometimes sophisticated bombers will include booby traps that will set off the bomb when someone tampers with it, like by cutting the decoy wires.
What percentage of bombs include such anti-tampering methods I couldn’t say, but it’s not zero, and you don’t want to take a chance. And as has been said, SOP isn’t to defuse the bomb, but rather to isolate it and make it go off.
Well, my electronics courses have long gone by, and I didn’t listen much in any case :o, however unless I completely miss my mark cutting out one part of a circuit, even a parallel dummy part which does absolutely nothing, would change the amperage in every other part. If the bomb trigger includes an ampere meter set to react to any sudden change, pop goes the bomb technician.
Or the bomb could be rigged to a *completed *circuit, and the trigger is that circuit going off (either by a programmed switch, or a bomb technician with a pair of wire cutters). Boom again.
I think Cecil did an article about this, but I’m too lazy to look it up.
(Scratches Smeghead off the “who to call in case of bomb threat” list.)
The best bomb defusing scene ever was in Scarecrow and Mrs. King.
MRS. KING: I think you should cut the blue wire. Yeah, the blue wire.
Scarecrow cuts the blue wire and the countdown stops.
SCARECROW: How did you know?
MRS. KING: Well, at home, whenever something goes wrong with the dishwasher, I just jiggle the blue wire.
I agree that it all depends on the bomber.
I also agree with an episode of MASH in which a bomb lands in the camp, but fails to explode. It turns out to be one of our bombs, so the people with appropriate clearance at ICOR have a manual describing how exactly to defuse it.
Another trope is putting the bomb in a refrigerator or stove to contain the blast. Mythbusters tried a grenade in a refrigerator. The explosion was more dangerous, because the fridge created vicious shrapnel.
MB’s did find that a bucket of water did a great job of containing the blast.
http://mythbustersresults.com/episode81
So forget about cutting wires. Fill the bath tub or sink. Put the bomb in and run like hell. 
Yes, in reality – but what kind of a movie scene would that make? For that matter, why would a bomber build a countdown timer display into his bomb in the first place?
[hijack] While we’re talking explosions. I remember Public Service Announcements warning kids not to pick up blasting caps. Kids didn’t know what they were and got hurt. Which kind of cap were they talking about? Most caps require crimping or electricity. Why would it go off in a kids hand? FWIW, I don’t plan on picking up any. ;)[end hijack]
PSA from 1970’s poor quality, but watchable
My favorite bomb-defusing cliche is the old:
Dude reading from manual: “Cut the green wire.”
Dude holding the tools and the bomb: “Okay.” <snip!>
Dude with manual: “But first…”
Dude with tools and bomb: “!!!”
But if you were inside the fridge, you could survive a nuclear blast!
Everyone knows IRL bombers make their bombs into elaborate Rube Goldberg machines. Sort of like that movie where Tommy Lee Jones was trying to blow away everyone and Jeff Bridges was trying to stop him and almost got blown away himself? I think it was called “Blow’d Up.”
Incidentally, some bombs can be disarmed by rigging a smaller outside charge at one end of a water tank. The pressure wave moves through the water and smashes the bomb to pieces faster than the detonator is activated. Takes a while to set up of course, but it’s a useful option in cases where it may be uncertain whether the bomb material is contaminated.
aceplace
Blasting caps have to have what’s called a primary explosive inside them. That means an explosive that will go off with heat, pressure, or shock. They were worried about kids picking them up and throwing them, dropping them or whatever. They make quite a bang and could easily damage someone’s hand severely.
I also remember the old ads. They always made me feel cheated that I never found any.
Testy
When my dad was in grade school the kid behind him was picking the powder out of a blasting cap to make a whistle out of it. The kid lost a thumb and two fingers. Dad ended up with hand parts over the back of his head and neck.
Helps setting the timer in the first place, no ? I would however agree that the loud beeping every second is unnecessary 
My guess is, it’s a holdover from the olden days when people did it with mechanical alarm clocks, using the ringing of the bell to make the electrical contact. Those seconds tick,tock,ticking away were great tension builders, weren’t they ?
If I were to design a bomb, making it difficult to disarm would be way down on my list of objectives. I’d spend more time thinking about ways to deploy it so it would be unlikely to be found before it was triggered. For my own safety, I’d probably make it very easy to disarm.
Does Hamas observe wiring color codes in the bombs they plant?