3.2 Protection Equip- Equip-ment for Low-Voltage
3.2.2 Switchgear Assemblies
Switchgear assemblies are series-connected switching and protection devices which perform specific tasks for protecting a system component;
the first device (relative to the flow of power) provides the short-circuit protection.
Switchgear assemblies with fuses Fuses and molded-case circuit-breakers
If the prospective short-circuit cur-rent Ikexceeds the rated short-circuit breaking capacity Icnof the circuit-breaker at its point of installation, the latter must be provided with up-stream fuses (Fig. 3/7).
Protection and operating ranges Defined protection and operating ranges are assigned to each device in the switchgear assembly. The L-re-lease monitors overload currents, while the I-release detects cuit currents up to the rated short-cir-cuit breaking capacity of the cirshort-cir-cuit- circuit-breaker.
The circuit-breaker provides protec-tion against all overcurrents up to its rated short-circuit breaking capacity Icnand ensures all-pole opening and reclosing. The fuses are only respon-sible for short-circuit clearance with higher short-circuit currents Ik. In this case too, the circuit-breaker discon-nects all-pole almost simultaneously via its I-release, triggered by the let-through current IDof the fuse. The fuse must, therefore, be selected such that its let-through current IDis less than the rated short-circuit breaking capacity Icnof the circuit-breaker.
Fuse, contactor, and thermal inverse-time-delay overload relay The contactor is used to switch the motor on and off. The overload relay protects the motor, motor supply conductors and contactor against overloading. The fuse upstream of the contactor and overload relay pro-vides protection against short cir-cuits. For this reason, the protection ranges and characteristics of all the components (Fig. 3/8) must be care-fully coordinated with each other.
The switchgear assembly comprising contactor and overload relay is re-ferred to as a motor starter or, if a three-phase motor is started directly, a direct-on-line starter.
Specifications for contactors and motor starters
The standards EN 60947-4-1 / IEC 60947-4-1 / DIN VDE 0660-102 apply for contactors and motor starters up to 1,000 V for direct-on-line starting (with maximum voltage).
When short-circuit current protection equipment is selected for switchgear assemblies, a distinction is made be-tween various types of protection ac-cording to the permissible degree of damage as defined in EN 60947-4- / IEC 60947-4-1 / DIN VDE 0660-1021):
Type a Destruction and replace-ment of individual compo-nents or complete switch-ing device
Type b Welding of contacts and permanent change in characteristic values of overload relay
Type c Welding of contacts with-out permanent change to operating values of over load relay.
Protection and operating ranges of equipment
Grading diagram for motor starter The protection ranges and the rele-vant characteristics of the equipment constituting a switchgear assembly used as a motor starter are illustrated in the grading diagram in Fig. 3/8.
t
L release I release Fuse Circuit-breaker Fuse IcnRated short-circuit
breaking capacity Ik Prospective
sus-tained short-circuit current at mounting location
A Safety margins
Fig. 3/7 Switchgear assembly comprising fuse and circuit-breaker
1) The standards EN 60 947-1 / IEC 60 947-4-1/
DIN VDE 0660-102 comprise modified de-scriptions for short-circuit behavior as follows:
Coordination type ”1”:
Destruction of contactor and overload relay are permissible. The contactor and/or over load relay must be replaced if necessary.
Coordination type ”2”:
The overload relay must not be damaged.
Contact welding at the contactor is, however, permissible, given the contacts can easily be
3
3/17
The fuses in this assembly must satisfy a number of conditions:
C The time-current characteristics of fuses and overload relays must al-low the motor to be run up to speed.
C The fuses must protect the over-load relay from being destroyed by currents approximately 10 times higher than the rated current of the relay.
C The fuses must interrupt overcur-rents beyond the capability of the contactor (Iecurrents approximately 10 times higher than the rated oper-ating current Ie of the contactor).
C In the event of a short circuit, the fuses must protect the contactor to such an extent that any damage does not exceed the specified de-grees of damage mentioned above (depending on the rated operating current Ie, contactors must be able to withstand motor start-up cur-rents of between 8 and 12 Ie with-out the contacts being welded).
To satisfy these conditions, the fol-lowing safety margins A, B and C must be maintained between certain characteristic curves of the devices:
Protection of overload relay In order to protect the overload relay, the prearcing-time/current character-istic of the fuse (an LV HRC
switchgear fuse of utilization cate-gory aM was used in this example;
refer to the following section ”Select-ing fuses”) must lie in margin A be-low the intersection of the tripping curve of the overload relay (1) with its destruction curve (2).
Protection of contactor In order to protect the contactor against excessively high breaking cur-rents, the prearcing-time/current characteristic curve of the fuse, start-ing from the current value which cor-responds to the breaking capacity of the contactor (3), must lie in margin B below the tripping characteristic of the overload relay (1).
In order to protect the contactor against contact welding, time-current characteristic curves can be specified for each contactor indicating which load currents can be applied as maxi-mum currents so that
C contact welding is avoided, or else C welded contacts can easily be
sep-arated (characteristic curve 4 in Fig. 3/8).
Therefore, in both cases, the fuse must respond in good time. The total clearance time characteristic of the fuse (6) must lie in margin C below the characteristic curve of the contac-tor for easily separable contact weld-ing (4) (total clearance time = prearc-ing time + extinction time).
Selecting fuses
LV HRC switchgear fuses
Fuses for motor starters are selected according to the aforementioned criteria.
Compared with LV HRC fuses of uti-lization category gL used to protect lines and cables, LV HRC switchgear fuses of utilization category aM pro-vide the advantage of weld-free short-circuit protection for the maxi-mum motor power which the contac-tor is capable of switching.
Owing to their more effective current limiting abilities (as compared with those of line-protection fuses), they are very effective in relieving contac-tors of high peak short-circuit cur-rents ipsince they respond more rapidly in the upper short-circuit range as shown in Fig. 3/9.
It is therefore preferable to use switchgear fuses rather than line-protection fuses with relay settings
> 80 A at higher operating currents with correspondingly lower short-circuit current attenuation.
Table 3/9 shows the classification of the fuses based on functional features.
t
I Assembly comprising LV HRC fuse, contactor, and
4 (Depends on current limiting
1 Tripping characteristic of (thermal) inverse-time-delay overload relay
2 Destruction character-istic of thermal overload relay
3 Rated breaking capacity of contactor
4 Characteristic of contac-tor for easily separable welding of contacts 5 Prearcing-time/current
characteristic of fuse, utilization category aM 6 Total clearance-time
characteristic of aM fuse
A, B, C Safety margins for reli-able short-circuit pro-tection
Fig. 3/8 Switchgear assembly comprising fuse, contactor, and thermal inverse-time-delay overload relay
Classification of LV HRC fuses and comparison of characteristic curves of gL and aM utilization categories
LV HRC fuses are divided into func-tional and utilization categories in ac-cordance with their type design. They can continuously carry currents up to their rated current.
Functional category g (full-range fuses)
Functional category g applies to full-range fuses which can interrupt cur-rents from the minimum fusing current up to the rated short-circuit breaking current.
Utilization category gL/gG This category includes fuses of uti-lization category g/gG used to protect cables and lines.
Functional category a (back-up fuses)
Functional category a applies to backup fuses which can interrupt currents above a specified multiple of their rated current up to the rated short-circuit breaking current.
Utilization category aM
This functional category applies to switchgear fuses of utilization cate-gory aM, the minimum breaking
cur-times the rated current. These fuses are thus only intended for short-circuit protection. For this reason, fuses of functional category a must not be used above their rated current. A means of overload protection, e.g. a thermal time-delay relay, must there-fore always be provided.
Comparison of characteristic curves for utilization categories gL and aM
The prearcing-time/current character-istics of LV HRC of utilization cate-gory gL and aM for 200 A are com-pared in Fig. 3/9.
Switchgear assemblies without fuses (fuseless design)
Back-up protection (cascade-con-nected circuit-breakers)
If two circuit-breakers with I-releases of the same type are connected in series along one conducting path, they will open simultaneously in the event of a fault (K) in the vicinity of the distribution board
(Fig. 3/10, 3/11).
The short-circuit current is thereby detected by two series-connected interrupting devices and effectively extinguished. As a result, the down-stream circuit-breaker with a lower
stalled at a location where the possi-ble short-circuit current exceeds its rated switching capacity.
Protection and operating ranges of the circuit-breakers
Fig. 3/10 shows the single-line dia-gram and Fig. 3/11 the principle of a cascade connection. The rated cur-rent of the upstream circuit-breaker Q2 is selected in accordance with its rated operating current. The circuit-breaker Q2 can, for example, be used as a main circuit-breaker or group cir-cuit-breaker for several feeders in sub-distribution boards. Its I-release is set to a very high operating cur-rent, if possible to the rated short-cir-cuit breaking capacity Icnof the downstream circuit-breakers.
ts Prearcing time for fuse
Fig. 3/9 Comparison of prearcing-time/
current characteristics of LV HRC fuses of utilization categories gL and aM, rated current 200 A Table 3/9 Classification of LV HRC fuses based on their functional characteristics defined in
EN 60269-1/ IEC 60269-1/DIN VDE 0636-10
Functional category Utilization category
Designation Rated continuous Rated breaking Designation Protection of current ≤ current
Full-range fuses
Ia min Minimum rated breaking current
Q2
Q1
Circuit-breaker with I-release
Circuit-breaker with L I-release and
K
3
3/19
The outgoing circuit-breaker Q1 pro-vides overload protection and also clears autonomously relatively low short-circuit currents which may be caused by short circuits to exposed conductive parts, insulation faults or short circuits at the end of long lines and cables. The upstream circuit-breaker Q2 is only involved at the same time if high short-circuit cur-rents occur as a result of a dead short circuit in the vicinity of the out-going circuit-breaker Q1 (restricted selectivity).
Circuit-breakers with L- and I-releases and contactor
Protection and operating ranges of devices
The circuit-breaker provides overload and short-circuit protection also for the contactor, while the contactor performs switching duties (Fig. 3/12).
The requirements that must be ful-filled by the circuit-breaker are the same as those that apply to the fuse in switchgear assemblies comprising fuse, contactor and overload relay (see Fig. 3/8).
Starter circuit-breaker with I-release, contactor, and overload relay (a)
Readiness for reclosing
Overload protection is provided by the overload relay in conjunction with the contactor, while short-circuit pro-tection is provided by the starter cir-cuit-breaker. The operating current of its I-release is set as low as the start-ing cycle will permit, in order to in-clude low short-circuit currents in the instantaneous breaking range as well (Fig. 3/13). The advantage of this switchgear assembly is that it is pos-sible to determine whether the fault
was an overload or short circuit ac-cording to whether, via the overload relay, the contactor or the starter cir-cuit-breaker has opened. Further ad-vantages of the starter circuit-breaker following short-circuit tripping are three-phase circuit interruption and immediate readiness for reclosing.
The switchgear assemblies with the starter circuit-breaker are becoming increasingly important in fuseless control units.
Switchgear assemblies with ther-mistor motor-protection devices Overload relays and releases cease to provide reliable overload protec-tion when it is no longer possible to establish the winding temperature from the motor current. This is the case with:
C High switching frequencies C Irregular, intermittent duty C Restricted cooling and C High ambient temperatures In these cases, switchgear assem-blies with thermistor motor-protec-tion devices are used. The
switchgear assemblies are designed with or without fuses depending on the installation’s configuration.
Temperature sensor in motor winding The degree of protection that can be attained depends on whether the motor to be protected has a ther-mally critical stator or rotor. The oper-ating temperature, coupling time con-stant and the position of the
temperature sensor in the motor winding are also crucial factors. They are usually specified by the motor manufacturer.
Fig. 3/11 Principle of a back-up circuit (cascade connection)
ip Maximum asymmetrical short-circuit current (peak value) iD1 Let-through current Q1 iD (1+ 2) Actual let-through current
(less than iD1)
ue Source voltage (opening voltage)
uB (1+ 2)Sum of arc voltages of up-stream circuit-breaker Q2 and outgoing circuit-breaker Q1 uB1 Arc voltage of outgoing
circuit-breaker Q1
Motors with thermally critical stators
Motors with thermally critical stators can be adequately protected against overloads and overheating by means of thermistor motor-protection de-vices without overload relays. Feeder cables are protected against short cir-cuits and overloads either by fuses and circuit-breakers (Fig. 3/14a) or by fuses alone (Fig. 3/14b).
Motors with thermally critical rotors
Motors with thermally critical rotors, even if started with a locked rotor, can only be provided with adequate protection if they are fitted with an additional overload relay or release.
The overload relay or release also protects the cabling against over-loads (Fig. 3/14a, c and d).