MV design guide - Schneider Electric

Presentation
Metal-enclosed,
factory-built equipment
Current
Rated normal current: Ir (A)
This is the rms. value of current that equipment can withstand when
closed, without exceeding the temperature rise allowed in standards.
The table below gives the temperature rises authorised by the IEC
according to the type of contacts.
Rated normal current:
Type of mechanism Max. values
of material
Max. temperature Max. temp. rise
of conductor (°C) = t°. max. - 40 °C
contacts in air
bare copper or copper alloy 75 35
silver or nickel plated 105 65
tin-plated 90 50
bolted connections or equivalent devices
bare copper, bare copper alloy
or aluminium alloy 90 50
silver or nickel plated 115 75
tin-plated 105 65
N.B.: rated currents usually used by Merlin Gerin are:
400, 630, 1 250, 2 500 and 3 150 A.
Examples:
c For a switchboard with a 630 kW motor
feeder and a 1 250 kVA transformer feeder
at 5.5 kV operating voltage.
v calculating the operating current
of the transformer feeder:
Apparent power:
S = UIe
Operating current: I (A)
This is calculated from the consumption of the devices connected to the
circuit in question. It is the current that really passes through the
equipment.
If we do not have the information to calculate it, the customer has to
provide us with its value. The operating current can be calculated when
we know the power of the current consumers.
S 1 250
I = = = 130 A
Ue 5,5 • 1,732
v calculating the operating current
of the motor feeder:
cosϕ = power factor = 0.9
η = motor efficiency = 0.9
P 630
I = = = 82 A
Uecosϕη 5.5 • 1.732 • 0.9 • 0.9
8 Merlin Gerin MV design guide Schneider Electric