Sous-titres (517)
0:00- On October 23rd, 2018,
0:02thousands of football
fans were making their way
0:05Excitement for the game was high,
0:07and the fans began to chant and sing.
0:11around 50 people were
riding the long escalator
0:14down to the platform,
but within 30 seconds,
0:16the crowd had swelled
to nearly double that.
0:19Everything seemed fine,
0:20but inside the escalator was a problem.
0:25The weight of the passengers
was bearing down on the steps,
0:28and the load on the main
motor was increasing.
0:31To try to slow the descent,
0:32the motor applied a counter torque.
0:35But as the force continued to increase,
0:36the stairs began to move faster.
0:39By 7:04 pm, the crowd had tripled.
0:42The motor finally reached its limit,
0:44and under the massive strain,
the drum began to slip.
0:47With the motor losing control,
0:49the escalator triggered
its second line of defense.
0:52A safety relay tripped,
0:54immediately cutting power to the motor.
0:56The main brake clamped
down on the metal drum
0:59to stop the descent of the stairs,
1:05The friction on the
main drum wasn't enough
1:08to stop the motor from spinning,
1:09and the stairs continued to accelerate.
1:12Sensing the motor had
lost control of the steps,
1:14the escalator engaged
the last line of defense.
1:17In the event of an emergency,
an auxiliary break is designed
1:20to bypass the motor entirely
1:22and directly lock the drive shaft.
1:25Under normal circumstances,
1:27the chance that all three
safety measures fail
1:29at the same time is vanishingly small.
1:33But these weren't normal circumstances.
1:35At 7:05 pm, the third and
final safety system failed,
1:39and the stairs began to plummet.
1:42Fans were flung forward
1:43and started streaming down the escalator.
1:46Some lept over the central
barrier in desperation,
1:49while others were swept
into a crushing pileup.
1:52At the bottom, the landing
became a dangerous choke point.
1:55Under the pressure, the steps twisted
1:57and buckled into jagged metal,
leaving 24 people injured.
2:01Something like this
shouldn't have been possible,
2:04and experts at the time knew
that something had gone wrong,
2:07and they started to suspect foul play.
2:10In the aftermath, Rome's Transit Agency
2:12sealed off the accident site
2:13and closed the Republica
station for several months.
2:16The authorities ordered both a technical
2:19and a criminal investigation,
2:20and the mayor even publicly vowed
2:22to discover the cause of the accident.
2:25So investigators began
dismantling the wreck,
2:28tearing it down piece by piece
2:29to reconstruct what had happened.
2:34The ride people experienced
on that escalator
2:36was one of the most terrifying
rides of their lives.
2:39But maybe it's more similar to the origin
2:42of escalators than you might think.
2:43Like what was the first
escalator even used for?
2:47Do you wanna have a guess?
2:51It was an attraction in a theme park
2:53all the way back in 1896.
2:55It had no steps, a 25 degree incline,
2:58and it was essentially
just a slow conveyor belt
3:01made of metal and wooden parts.
3:03It brought people up a full seven feet
3:05before they would have to walk
downstairs on the other side,
3:09and it was a huge success.
3:11Over 75,000 people enjoyed the attraction
3:14during its two week
stay at the Old Iron Pier
3:18The ride was named the Continuous Elevator
3:20and its inventor, Jesse
Reno, had created it
3:23not just as an attraction,
but as a proof of concept,
3:27because he saw it as the
future of transportation.
3:33But as Reno watched
people ride his invention,
3:36he began to notice a pattern.
3:39Instead, they stood still,
feet planted firmly sideways
3:43with people gripping the handrail tightly.
3:47Two years later, the department
store Harrods in England
3:49installed a similar device,
3:51but the ride was so
unsettling that Harrods
3:53had to put staff at the
top to offer brandy to men
3:56and smelling salts to women
just to calm their nerves.
4:00You see, for both devices,
4:01the 25 degree conveyor belt
was precarious to walk on
4:05and unnerving to stand on.
4:07At around 12 degrees,
4:08walking on an incline becomes difficult,
4:10and 25 degrees is roughly the limit
4:13that our ankles can flex.
4:16If only there was a way to
replace the conveyor belt
4:18with a moving set of stairs.
4:20Well, then the riders would
always have a flat surface
4:23to stand on and a
staircase they could climb
4:28One attempt at a solution
had already been around
4:30for four decades, and it was
called the revolving stairs.
4:34It consisted of a chain
that went around a loop,
4:36then fixed stair shaped
blocks were attached to it,
4:39creating a flat surface to stand
on during the main incline.
4:42But as soon as you'd reached the top,
4:44the steps tilted forward, making
it treacherous to get off,
4:47and a similar problem
plagued you at the bottom.
4:51Now, you might think if the top
4:52and bottom are causing problems,
just extend each landing,
4:55but that also doesn't work.
4:57You just end up with a
jagged mess for longer.
5:00So how do modern escalators
solve this problem?
5:04I mean, have you ever
stopped to think what happens
5:06to the stairs at the top
5:07of the escalator when they disappear?
5:10Clearly, we have steps going
around in some sort of loop,
5:13but how do they actually
behave on the return journey?
5:16What if I give you two options?
5:18Do they stay right side up like the cabins
5:20in a Ferris wheel, or
do they flip upside down
5:23and then flip back
again at the other side?
5:26- I'm gonna go this one all day.
5:28- Ferris wheel.
- This one makes more sense.
5:32- I think they turn upside down.
5:34- And they're actually right side up.
5:35- I think I'm gonna go with this.
5:39- They just keep-
- Yeah.
5:39- The solution to this problem
came from another inventor
5:41named George Wheeler.
5:43His idea forms the basis of
every escalator in use today.
5:47A modern version of it
works something like this.
5:50A typical subway escalator has
an electric motor at the top
5:53with a power output of
around 50 kilowatts.
5:56Smaller than most electric cars.
5:59This motor spins extremely
fast at over a 1,000 RPM,
6:02but it's pretty weak.
6:04So to drive the steps, the escalator needs
6:06to convert this into a slower
output with more force.
6:10To do this, it uses a reduction
gearbox in a gear system,
6:13lowering the output to just a few RPM
6:15and increasing the torque
by a factor of around 100.
6:20The motor is connected
with a large sprocket
6:22to a reinforced steel chain,
6:24which pulls the stairs around a loop.
6:26The so-called step chain
is fitted with wheels
6:28to allow it to roll
smoothly around curves.
6:32But unlike the design
for the revolving stairs,
6:34Wheeler proposed attaching
each step to this chain
6:37through a single axle, giving
it the freedom to rotate.
6:41Next, he added a second
set of wheels to each step
6:43that followed a different track,
6:45allowing him to control the
angle of each step at any point.
6:49On the incline, the two tracks overlap
6:52just like the revolving staircase,
6:53but then at the top,
the two tracks separate,
6:56and this is what allows us
6:57to keep the steps level
throughout the entire ride.
7:01The tracks then remain
separated and curve around.
7:04The steps flip upside down,
7:05tuck into the loop, and
start their return journey.
7:08At the start of the
incline, the tracks rejoin
7:10and the whole process repeats.
7:13- So the answer is, you are both wrong.
7:20- I don't want to be interviewed anymore.
7:21- Yeah.
- Wow. I never really thought about that.
7:23- Yeah, like, I would say,
7:24it's like a upside down elevator, bro.
7:27- Guess what? You're right.
7:32- But despite all modern escalators
7:34adopting Wheeler's design, at the time,
7:36it caught so little attention
7:38that he was forced to shelve the idea.
7:40It wasn't until eight years
later that another inventor,
7:43Charles Seeberger, bought his patent
7:45and capitalized on the invention.
7:47Seeberger partnered with
the Otis Elevator Company,
7:50and together they built a prototype.
7:53A year later, in 1900,
7:55they showcased it at the
Paris Exposition Universelle.
7:59In total, 51 million people
flocked to the exposition
8:02to see the marvels of modern technology.
8:04But one of the most popular exhibitions
8:06was the world's first
true commercial escalator.
8:09The machine drew huge crowds.
8:11French historian Philippe
Jullian described it
8:13as the jolliest attraction
at the exhibition
8:15and wrote, "The escalator
caused many an incident worthy
8:19"of the vaudeville, separating families,
8:21"sending old men sprawling,
delighting the children,
8:23"and reducing their nannies to despair."
8:26The escalator was even awarded
one of the grand prizes
8:30Shortly after, escalators
started being installed
8:32in different places across the world.
8:35But these escalators weren't perfect.
8:38They had smooth flat stairs,
8:40and when they reached the top,
8:41these stairs would disappear
under a wooden board,
8:44leaving a dangerous gap between them.
8:47Shoe laces, coats, and
especially the long skirts
8:49in fashion at the time easily
got caught in the machinery.
8:53One incident even saw a three year girl
8:55getting her foot pinched in the gap.
8:57And while the girl luckily
escaped with injured toes
9:00and a missing shoe, something
in the design had to change.
9:04To solve this, Seeberger
9:06and Otis installed a
triangular shunt at the end
9:09of the escalator, forcing
rider to go off to the left
9:12before they reached the dangerous gap.
9:14This system worked, but it was awkward
9:16because it meant people
had to put one foot
9:18onto solid ground while
the other was still moving,
9:21which became especially tricky
9:22when some people stood
still and others walked.
9:25So to reduce the risk of people
getting in each other's way,
9:28operators asked people
to stand on the right
9:31and keep the left lane
clear for faster walkers.
9:34It's a convention we still
often follow to this day,
9:39there's a much better
solution than the shunt.
9:43Modern escalator steps aren't
smooth, they're grooved.
9:46These grooves then interlock perfectly
9:48with a comb plate at the
top of the escalator.
9:51So now, if a small item
approaches the end,
9:54the comb plate lifts it
up and out of harm's way.
9:57This makes it much harder
for things to get stuck,
9:59and perhaps more importantly,
10:00it allows people to
safely step off forwards.
10:05But the comb plate doesn't
entirely solve the problem.
10:08We still have these gaps on
the side of the escalator
10:10that can pinch and trap
objects as the steps move.
10:14a new safety feature called
the skirt brush was added
10:17to the escalator in 1982.
10:21Escalators are full of subtle
safety features like this,
10:24some old and some new,
10:25but almost all of them are
designed around people.
10:28All the way back in 1896,
Jesse Reno predicted
10:31that riders on his attraction
would need something
10:33to hold onto, so he
introduced a moving handrail.
10:36In a modern escalator, the
motor has a separate connection
10:39to turn a friction wheel
that drives the handrail.
10:43The only problem is that
friction wears things down.
10:46So over time, the wheel gets smaller,
10:48and as its circumference decreases,
10:51each rotation moves the
rubber loop a slightly shorter
10:54distance, so the handrail
begins to move more slowly.
10:58The effect is small, but
it builds up over time.
11:01So to compensate for this,
a new handrail is calibrated
11:04to move around 2% faster than the steps.
11:07You can actually try this yourself.
11:09Next time you're standing on an escalator,
11:11just place your hand next
to you as you stand still,
11:14and you will watch as your
hand slowly drifts forward.
11:18This speed difference stops the handrail
11:20from lagging too far
behind the steps over time.
11:23- Because I have definitely noticed that,
11:26that sometimes I'm on an escalator
11:28and then it's going faster than me.
11:30My hand is going faster than my body,
11:34but that means it's a new escalator.
11:36- Well, it's a new frictional wheel.
11:38That wheel that drives the
hand, we don't replace,
11:43we don't replace the entire escalator.
11:45- Oh, wow, so that's like
a party trick I can use
11:49to entertain my friends.
11:50I mean, I don't know when I'd
have a party on an escalator,
11:52but whatever, if I'm on an
escalator with my friends
11:55and I can see it moving, I'd be like,
11:57"Hey, that's 'cause there's
a new frictional wheel."
12:00I can tell them that and impress them.
12:03- But it's not just the handrail.
12:06The speed of the steps
themselves is also something
12:09that needs to be carefully controlled.
12:10And modern escalators
use AC induction motors,
12:13which are extremely good at regulating
12:15their rotational speed.
12:17And this has an unexpected
benefit on downward escalators.
12:22With enough people riding,
their weight is enough
12:24that the motor no longer
has to power the ride.
12:27Instead, the weight of
the passengers themselves
12:30drives the chain and
causes the motor to spin.
12:33As more people board, the
force on the motor increases,
12:36and it's pushed to turn faster.
12:38But modern AC induction motors work
12:40by creating a rotating magnetic field.
12:42When the motor tries to
spin faster than the field,
12:45electric currents are induced inside it,
12:47which then create their
own magnetic field.
12:49This new field pushes back
in the opposite direction
12:52to the spin, creating a braking force,
12:54which resists the increase in speed.
12:58But something interesting
happens when the motor resists
13:00like this, rather than consuming energy,
13:03the physics of the motor flips
13:05and it uses the excess mechanical energy
13:07to produce an electric current.
13:09This is called regenerative braking,
13:11and it's the same trick
that electric vehicles use
13:13to recharge their batteries.
13:15In effect, the motor
turns into a generator.
13:18The result is that on a busy day,
13:20many modern downward escalators
aren't just moving people,
13:23they're actually generating electricity.
13:26Often this is channeled back
to the building's internal grid
13:29and used to power other devices,
13:31including the upward escalators.
13:32- So even the escalator
that was invented
13:39and was installed in 1920,
the Paris Exhibition,
13:44et cetera, I mean, all these
escalators were regenerators.
13:48- Yeah, when there were people standing
13:49on the escalator in down direction,
13:52these escalators were feeding
energy back into the grid.
13:55- No, it's like the down
escalator's a generator?
13:59- This regenerative braking
14:02makes escalators
extremely power efficient,
14:04but more importantly,
14:05it makes them inherently safe.
14:08But there is a point where
if you keep adding weight,
14:11then eventually the
force becomes so strong
14:13that the motor can no longer resist it.
14:15And if left unchecked,
14:16it would start accelerating
uncontrollably.
14:19The stairs would go plummeting down,
14:22which is exactly what happened in Rome.
14:25After a nearly two year
long investigation,
14:28the investigators published
this 86 page report.
14:31Inside it lists the
exact sequence of events
14:34that led to the disaster.
14:36As fans crowded onto the escalator,
14:38their combined weight increased
the load on the main motor.
14:41The motor tried to resist this change,
14:43but as more and more people funneled on,
14:45the force got too high,
14:46and eventually it hit a tipping point
14:48and the motor started
accelerating uncontrollably.
14:52Safety sensors in the machine
noticed this sudden change
14:55and triggered two things
in short succession.
14:57At first, the power to the motor was cut,
15:00and immediately after that,
the main brake engaged.
15:03Two massive arms clamped down on the drum
15:06to lock it in place and avert a runaway.
15:10This break should have
had enough stopping power
15:13to bring the fully loaded
escalator to a halt,
15:15even under the massive
strain, but it didn't.
15:19Tests after the incident showed
15:20that its braking force was
far too low, around 37%
15:24of the manufacturer's specification.
15:27The weakened brake struggled
to slow the spinning motor
15:30and the escalators downhill
acceleration continued.
15:34This is when the last
line of defense kicked in.
15:37When the escalator speed
rose by more than 20%,
15:40the auxiliary brake triggered
driving steel wedges
15:43into a disc on the drive shaft.
15:46But when investigators
opened up this brake,
15:49The final mechanical backstop
had been partially disabled.
15:53Someone had physically tied plastic straps
15:56around one of the two brake
wedges and rendered it useless.
16:02(dramatic music)
With half the system unable to engage,
16:04its stopping power was cut by 50%,
16:07just enough for the weight
of all those passengers
16:10to overpower the brake
16:11and render the last
line of defense useless.
16:15Investigators knew that these failures
16:17should have been automatically recorded
16:21but when they went to
check, they found nothing.
16:25The error codes had been turned off.
16:27Meaning critical malfunctions could occur
16:29without leaving a trace.
16:30The only way this could happen
was if they had been disabled
16:33on purpose, meaning someone
must have reprogrammed
16:36the system to stop recording fault codes.
16:40Next investigators turn to
the maintenance records,
16:44but they found these similarly incomplete
16:46and evidence of major
work on the escalator
16:48was nowhere to be found at all.
16:52With all the main safety
systems compromised
16:54and critical alerts turned off,
16:56the escalator had been
a ticking time bomb.
16:59All findings from the
technical investigation
17:01pointed not to a manufacturing defect,
17:04but to a pattern of neglect
17:05and falsification by those in charge
17:07of keeping the machine safe.
17:09This left the prosecution
with one clear question,
17:13who was responsible?
17:16The trail of evidence led back
17:18to June, 2017 when
maintenance responsibilities
17:21for Rome's escalators shifted
17:23to a new contractor, Metro Roma.
17:26The Transit Authority
ATAC severed its contract
17:28with Metro Roma in an attempt
17:30to wash its hands of the situation.
17:32But as the criminal inquest
continued, it became clear
17:34that the problem went far deeper.
17:37The investigators discovered
17:38that Metro Roma had been
working hand in hand
17:41with the Transit Authority ATAC,
17:43and together they presided
over negligent maintenance
17:46and falsified records
all across the network.
17:49By September 2019, 11 suspects were named
17:52and the courts had suspended
three ATAC managers
17:55along with the chief of Metro Roma.
17:58The prosecution's findings were grave.
18:02safety devices had been
deliberately sabotaged
18:04to avoid escalator shutdowns,
18:06and those in charge had
covered their tracks
18:09through a pattern of
fraud and obstruction.
18:11In the midst of the public outrage,
18:13prosecutors recorded a chilling wiretap
18:15of ATAC Manager Renato Domico.
18:18The translation, "If you run the numbers,
18:20"out of 700 escalators,
there'd be like three
18:23"or four more dropping. Come on."
18:25The prosecutors note in their report
18:27that Domico appeared
uninterested in the possibility
18:30there might have been people
18:31on those three or four escalators.
18:33It was simply a matter of
numbers and percentages to him.
18:37It was a callous remark
18:38and it painted a clear
picture of the incident.
18:41This wasn't an engineering
failure, it was a human one.
18:47But that brings us to a
more fundamental question.
18:49I mean, how safe are escalators really?
18:52The truth is when they're
properly maintained,
18:54the safety margins on
escalators are enormous.
18:57Each system is engineered
to handle forces far beyond
19:00what they'll ever see in service.
19:02- So the breaking load
19:03of our step is like greater
15 kilonewtons to 1.5 tons.
19:08So you can put an elephant on
the step and it won't break.
19:12Well, I've never seen a step
break in my work career.
19:16Never seen a step chain break either.
19:18I mean, it's does not happen.
19:21I mean, I'm not here to say
that there are no accidents
19:24on an escalator, but the accidents I know,
19:29I mean, it's critical like that you,
19:32that you ensure the right maintenance.
19:35That's the most important thing,
19:37because in the end, it's
all about maintenance.
19:41- When this is done right,
19:43the chances of a catastrophic
failure are vanishingly small
19:47and with around 1.5 million
escalators worldwide,
19:50that really is how it should be.
19:52In the US and Canada alone,
19:54over a hundred billion escalator trips
19:57are happening every year,
19:58making the escalator one of
the most widely used forms
20:01of transport on the planet.
20:03On a scale that large, it's sometimes easy
20:05to point the finger at our
technology when things go wrong.
20:09no matter how well
designed our systems are,
20:12they all rely on people to maintain them.
20:14And perhaps that's the lesson here.
20:17As humans, we have a duty of
care, not just to ourselves,
20:20but to everyone around us.
20:23And sometimes that means
taking responsibility
20:25for keeping each other safe.
20:33In a way, that's how the
escalators story began
20:36with one person deciding
to take responsibility
20:38for a problem that everyone else ignored.
20:41Back when Jesse Reno was
at university, every day,
20:44he had to climb more than
300 steps to get to his frat house.
20:48But while everyone else
complained about this,
20:50Reno did something about it.
20:52He had the math, the science,
20:53and most importantly, the
problem solving skills
20:56to create the world's
very first escalator,
20:58which he took to Coney Island.
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