In the case of motor we can multiply HP value with 1.43.

Wikipedia has the answers to both questions.

http://www.wikipedia.org/

<...calculate current from HP ...>

Convert to kW and divide by voltage, usually.

<...When do arcs flashes typically occur? What are ways to prevent?...>

Turn the voltage down?

1. How do you calculate current from HP (Horse Power)?

2. When do arcs flashes typically occur? What are ways to prevent?

1. To calculate Current from Horse Power:

1. Multiply by 0.746. [746 watts is equal to 1 Horse Power]'
2. The unit of the result got is kW [kilowatts].
3. Note the number of Phases in the nameplate connection diagram [say 3 phases].
4. Note the value of the nameplate voltage [ kV].
5. Note the power factor if it is given in the nameplate if not take it for a motor as 0.82 lagging.
6. Note the efficiency if given in the nameplate if not take it for a motor as as 0.87

Now the Current is = (HP) (0.746) (3) ^-½ (kV) ^-1(Power factor) ^-1(Efficiency) ^-1 Amps.

2. Flashes typically occur

a) Dielectric capacity falls due to humidity, dust, dirt etc.

b) Voltage stress increases due to Sharp bends or Points projected from surfaces.

c) Body of insulation develops voids i.e. air pockets are formed.

d) Spaces like blow holes remains inside Moulds / Casts of support insulators.

For Prevention

a) Abnormalities in construction are to be physically checked.

b) Materials are to be tested voltage withstand capacity.

c) Noises during Voltage tests are to be recorded.

d) In commissioned systems Insulation testing and Ground leakage testing is to be done in specified periods. In Unearthed systems Groung leak detectors are to be incorporated.

This is a very good answer! Just wanted to let you know ^_^