Siemens BETA Low-Voltage Circuit Protection

Appendix

Glossary

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Siemens ET B1 · 10/2008

Operational classes

Operational classes for low-voltage fuses are designated by let-

ters, the first of which represents the function class and the sec-

ond the object to be protected. The objects to be protected are

specified as follows:

letter

This produces the following operational classes:

Operational voltage

The voltage between two lines respectively on any equipment or

system component.

Overcurrent

Every current in a circuit that exceeds the value of the rated cur-

rent; overload, short-circuit current. Overcurrent can damage

lines, electrical machines and devices due to unacceptably high

heat generation and the possible occurrence of mechanical

forces

Overload

Operating conditions that cause an overcurrent in an electrical

circuit that is undisturbed and fault-free. Overload can cause

damage when it is sustained over an extended period and not

switched off.

Overload current

The overcurrent that occurs in a circuit that is undisturbed and

fault-free.

Overvoltage U

Peak value of the high-frequency transient recovery voltage; it is

greater than the rated operational voltage U

(

> U

)

Peak short-circuit current

The highest instantaneous value of the prospective current after

the short-circuit has occurred. It is made up of the short-circuit

current and a direct current component and is specified as the

peak value.

PEN conductor

Grounded neutral conductor with protective function in three-

phase system.

p.f.

Power factor: The ratio of active power to apparent power in AC

and three-phase systems.

p.f. correction

Measure to increase the cost-effectiveness of electrical sys-

tems. In doing so it corrects (compensates) the reactive power

generated in the inductive loads during operation. In a p.f. cor-

rected system, the current in the infeed is smaller than that of an

uncorrected system. This enables the use of smaller conductor

cross-sections and reduces the cross-section of the winding

wires of transformers and generators. Depending on the appli-

cation, a distinction is made between: Individual, group and

central p.f. correction.

p.f. correction unit

This comprises capacitors that can be connected and discon-

nected to compensate the changing reactive power in systems.

Power

physical

The energy expended per time unit in Watts.

Electrical

The electrical energy consumed from the system by a load per

time unit.

Depending on the load, the following types of power occur in an

AC system:

•

Active power (P)

Part of the power that is converted to a different form of en-

ergy.

The proportion of active power to apparent power is calculated

as follows:

(

W) =

p.f.

•

Reactive power (Q)

Part of the power that cannot be converted to the desired en-

ergy form. Reactive power is created as a result of induction,

whereby the current and voltage curve are offset.

The proportion of reactive power to apparent power is calcu-

lated as follows:

(

var) =

sin

•

Apparent power (S)

This is the power taken from the system, which is a vectorial

combination of active and reactive power.

The apparent power is calculated as follows:

S (VA) = U * I

These three types of power are geometrically interdependent.

This can be illustrated on a power chart.

The relationship between active power and apparent power is

determined by the power factor (p.f.). It is expressed through a

trigonometric function as the cosine of the angle .

Protection against direct contact

Measures to protect persons (and domestic animals) from the

dangers resulting coming into contact with live components of

an electrical device.

This may be full or partial protection. In the case of partial pro-

tection, protection is only given in the case of inadvertent

contact.

Protection against indirect contact

Measures to protect persons (and domestic animals) from the

dangers resulting from a fault when coming into contact with

frames or other extraneous conductive components.

Protective conductor / PE conductor

Conductors used for some protective measures against indirect

contact for the bonding connection of frames with other frames

and extraneous conductive components that do not belong to

the electrical installation: e.g., ground electrodes, ground con-

ductors and grounded live components, PEN conductors of the

grounded terminal of the power source or an artificial neutral

point.

Protective measures

Measures for the protection of persons and animals against

touch voltages, resulting from insulation faults in electrical

devices.

PTC thermistor sensor

Component of the thermistor protective device that is installed in

the winding of the motor to be protected or in the cast resin

transformer. As the temperature rises, the PTC thermistor detec-

tor increases its resistance within a small specified temperature

range by several powers of ten.

Radial network

The wiring diagram corresponds to the branches of a tree,

whereby the supply comes from the trunk and there is only one

route (branch) for the electrical energy to reach each load (leaf)

(