Siemens BETA Low-Voltage Circuit Protection

Appendix

Glossary

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Siemens ET B1 · 10/2008

r.m.s. value

Value, which corresponds to the r.m.s. value of a periodic quan-

tity. Because the values in an AC system are instantaneous val-

ues that are dependent on the phase relation, current and volt-

age are specified in the theory of alternating currents as r.m.s.

values. Measuring devices (moving coil instruments) always dis-

play the r.m.s. value. For the sine curve, the

r.m.s. value = 1 /

Amplitude (peak value).

The r.m.s. value of an alternating current is the same as that of a

direct current, which generates the same heat (power) as the al-

ternating current.

Safe isolation

Safe isolation is ensured when an individual fault does not cause

the overspill of the voltage of one circuit according to another.

This is particularly important when safety extra-low voltages

(

SELV) and other voltages up to 1000 V are transmitted in a sin-

gle device.

Safety extra-low voltage (SELV)

Protective measure through which circuits with rated voltages up

to 50 V AC voltage or 120 V DC voltage can be operated without

grounding and the supply from circuits with higher voltages is

safely isolated.

Selectivity

Co-ordination of series-connected overcurrent protective devic-

es (miniature circuit breakers, fuses) for sequenced switch-off. It

must switch off the overcurrent protective device closest to the

point of short-circuit. The other overcurrent protective devices in

the busbar run remain switched on. Selectivity minimizes the ef-

fect of a fault both spatially and time-wise.

Service life

The period that electrical equipment works problem-free under

normal operating conditions. It is specified in (for

example) operating hours or switching cycles. A distinction is

made between mechanical and electrical service life.

The end of the electrical service life is reached, for example,

when the number of operating hours are exceeded or when the

switching contacts are worn out through contact erosion that re-

liable and safe contact can no longer be ensured.

The mechanical service life is specified by the manufacturer as

the number of switching cycles without load that the device can

execute without the need for servicing or the replacement of

parts.

Short circuit

Connection with negligible small impedance between two con-

ductors that are live under operating conditions. In this case, the

current is a multiple of the operational current; this can cause a

thermal (rated short-time current) or mechanical (peak withstand

current) overloading of the electrical equipment and system

components.

Short-circuit current (sustained short-circuit current

)

Overcurrent that occurs in an electrical circuit with negligible im-

pedance between points with different potentials, in the event of

a fault, e.g. with short-circuited terminals on electrical equip-

ment or in the case of defective jumpering.

Short-circuits to frame

A conductive connection between the frame and live compo-

nents of an electrical device due to a fault.

Short-circuit strength

Resistance of an electrical device against the electrodynamic

(

dynamic short-circuit strength) and thermal (thermal short-cir-

cuit strength) stress that occurs in the event of a short-circuit.

The characteristic for the dynamic stress is the peak short-circuit

current as the highest instantaneous value of the short-circuit

current.

The characteristic for the thermal stress of the short-circuit cur-

rent is the r.m.s. value of the short-circuit current for its duration

(

sustained short-circuit current).

Short-circuit voltage U

(

)

Rated value of the voltage in % of the rated input voltage that

needs to be applied to the input terminals of a transformer, so

that if the output terminals are short-circuited, the rated output

current (I

)

flows, relative to 20 °C.

Starting current

The current that occurs immediately after closing a circuit,

e.g. when connecting a transformer or motor. The size depends

on the instant of closing (phase relation of the voltage). Maxi-

mum current flows when switching on at voltage zero; it then de-

creases to the rated value after approx. 20 ms.

Surge current withstand capability

The peak value of the current that a circuit or switching device

can withstand in closed position under prescribed applications

or corresponding use.

Switching capacity

Current that can switch a switching device on and off under pre-

scribed conditions (making and breaking capacity).

If a switching device can switch on larger currents than it can

switch off or vice versa, the specifications of the switching ca-

pacity can be separated into making and breaking capacity

(

short circuit, making and breaking capacity).

Switchgear combination

Combination of l.v. switching devices, designed and tested ac-

cording to DIN VDE 0660 Part 500. This standard distinguishes

between type-tested (TSK) and partially type-tested (PTSK) l.v.

switchgear combinations.

TSK: the TSK is a switchgear combination that corresponds to

the switchgear combination type-tested in accordance with the

aforementioned standards without any significant deviations

from the original type or system.

PTSK: the PTSK is a switchgear combination that contains both

type-tested and non-type-tested components that can be shown

to conform to the respective testing requirements.

Switching frequency

Switching frequency specifies how many switching cycles can

be carried out with the switching device in a specified time peri-

od (e.g. in 1 h) under field conditions.

Switching operation

The result of actuating from one switching position to another

and back to the original position and, where applicable, passing

through all intermediate positions

Note:

This could be closing followed by opening. Actuations that do

not form a switching operation are called a series of actuations.

Switch / switching device

Device for switching on or interrupting current in one or more

circuits.

Temperature limit

The highest temperature that individual components of an elec-

trical equipment can withstand continuously without suffering

damage. It is reached by adding the ambient temperature and

the permissible self-heating temperature for the electrical equip-

ment. The functional safety of the device is at risk if the temper-

ature limit is exceeded.

Temperature-rise limit

The specified maximum overtemperature for the individual com-

ponents of an electrical device (temperature rise).

Temperature sensor

Semiconductor device that changes its resistance value de-

pending on the temperature. It serves to monitor the winding

temperature, e.g. in three-phase asynchronous motors (ther-

mistor protection) and cast resin transformers.

Thermistor protection

Protection of the motor or cast resin transformer through temper-

ature sensors fitted in the windings (PTC or NTC thermistors).

These directly monitor the winding temperature.