Siemens BETA Low-Voltage Circuit Protection

Appendix

Glossary

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Siemens ET B1 · 10/2008

Ambient temperature

The temperature of the air surrounding an electrical device, de-

termined under prescribed conditions (e.g. for enclosed minia-

ture circuit breakers; the temperature of the air outside the en-

closure). The ambient temperature affects the heat dissipation,

so that it may be necessary to reduce the rated current.

Apparent power

In the case of alternating current, the product of voltage and cur-

rent, without taking into account the power factor p.f. The appar-

ent power is a measurable variable. It is specified in VA (power).

Auxiliary circuit

All conductive components of a switchgear combination in a

circuit (with the exception of the main circuit), that serve for

control, measurement, signaling, latching, data processing, etc.

Back-up protection

Interaction of two coordinated, series-connected overcurrent

protection devices at points where a device (e.g. miniature cir-

cuit breakers) cannot switch the prospective short-circuit cur-

rent alone in the event of a fault. If a high short-circuit current oc-

curs, the backup overcurrent protective device relieves the

downstream device, thus prevent it from being subject to over-

load. Both protective devices must have sufficient switching ca-

pacity.

Body (of electrical device)

Exposed conductive component of an electrical device that is

not normally live, but which may be live in the event of a fault.

Breaker mechanism

Component of a key-operated switching device that locks the

switch in the ON position. The breaker mechanism includes all

the mechanical parts required for switching and latching to a

closed component. It also contains the release-free mechanism.

Breaking capacity

r.m.s. value of the current at a specified p.f. and specified volt-

age, which can still safely switch off a switching device or fuse

under prescribed conditions (rated breaking capacity) In the

case of alternating current, the r.m.s. value of the symmetric

components applies.

Breaking current

Current in one pole of a switching device or of a fuse at the

moment the arc is formed (in the case of alternating current, this

is the r.m.s. value of the AC components).

Central p.f. correction

p.f. correction units are primarily used for central p.f. correction.

These are assigned directly to a switchgear assembly and in-

stalled centrally.

Circuit breakers

General key-operated switching devices that switch on, control

and switch off currents in circuits under normal operating condi-

tions. Under prescribed conditions that are not normal through

to short circuit, they can also switch on the current, control it for

a specified interval and interrupt it.

Note:

Miniature circuit breakers are generally designed for infrequent

switching, although some versions are also suitable for more fre-

quent switching.

Circuit (electrical)

A circuit comprises all the electrical devices in an installation

that are protected by the same overcurrent protective device(s).

Clearance

The shortest (thread measure) distance between two conductive

components. It is decisive for the level of insulation of an electri-

cal device.

Conductive component

Component through which current can flow, even if it is not nec-

essarily used to carry currents during normal operation.

Contact

Component of the contact system over which the circuit is

opened or closed. A distinction is made between fixed and ad-

justable contacts and main and auxiliary contacts.

Contactor

Switching device with only one off position, usually without me-

chanical lock, which is not operated manually and which, under

normal conditions, can switch on, transmit, and switch off the cir-

cuit, including normal overload currents. Contactors are prefer-

ably used for high switching frequencies. A distinction is made

between contactors for switching motors and auxiliary contac-

tors for control.

Note:

A contactor can switch short-circuit currents on and off if de-

signed for this purpose. It is not generally designed for separa-

tion. A contactor whose main contact elements are locked in the

OFF position is described as a "rupteur" in French. There is no

comparable term in English.

Control circuit

This is part of an auxiliary circuit and covers all components of a

circuit that do not belong to the main circuit. Control circuits are

circuits for:

•

Signal generation and signal input,

•

Signal processing, including conversion, storage, locking and

amplification,

•

Signal output and the control of actuators and signal

transmitters.

Creepage

Current that forms on a surface that is insulating material when

dry and clean, due to conductive soiling between two live com-

ponents.

Creepage distance

The shortest distance the length of an insulating surface be-

tween two reference points (taking into account possible

grooves) along which a current can flow (rated insulation

voltage).

Current limitation

The expected peak short-circuit current, taking into account the

circuit constants (R, L) does not occur, but is limited to a lower

value; the let-through current. This is achieved by means of fus-

es or so-called current-limiting circuit breakers and instanta-

neous switches that switch off extremely quickly in the event of

high short-circuit currents (in a couple of milliseconds).

Current limiter

Circuit breakers and miniature circuit breakers whose contac-

tors are directly opened dynamically in the event of a short cir-

cuit without waiting for the release through the breaker mecha-

nism. The resulting arc considerably reduces the current.

The current limitation effect is achieved by the fast opening of

the contactor as the short-circuit current is rising and through the

outstanding efficacy of the arc quenching device. The electric

arc must be extinguished before the natural (prospective) cur-

rent reaches its peak if a limiting effect is to be achieved.

DC component

The largest of the deviations from the zero line of the normal si-

nusoidal oscillation of the AC current. Occurs briefly,

for example, in the event of a short-circuit. The DC component

can be no greater than 100 % of the peak value of the symmet-

rical short-circuit current and in the usual low-voltage systems is

usually a maximum 50 %.