Answer:
Electrical breakdown.
Explanation:
When two conductors are relatively close enough, and have a very large voltage between them, it can lead to a Dielectric breakdown. A dielectric breakdown occurs when an insulator is subjected to a high enough voltage, suddenly becomes an electrical conductor and electric current flows through it. The air between the conductors is the insulator that breaks down, leading to an electrical discharge arc to flow between the two conductors. This electrical breakdown can cause catastrophic failure of electrical equipment, and fire hazards.
The term pressure refers to the force per area created by the weight
of anything whose weight is distributed over an area, such as the
Earth's atmosphere, a lake, a gas inside a sealed jar, or a pointy
high heel.
We use the following expression
T = 2*pi *sqrt(l/g)
Where T is the period of the pendulum
l is the length of the pendulum
and g the acceleration of gravity
We solve for l
l = [T/2*pi]² *g = [30s/2*pi]²* 9.8 [m/s²] = 223.413 m
The tower would need to be at least 223.413 m high
See the attached picture: