Switchgear Icw and Icu of Circuit Breaker 1

[PoliPoliCarps]

Hello guys,

Can I ask, if should the Icw of the Switchgear be the same as the Icu of the Circuit Breaker.
For example I will use Icw=85kA for the LV Switchgear, should I use 85kA for each protective devices inside that LV Switchgear?

Thank you in advance

 

[che12345]

Deaar Mr PoliPoliCarps

Q. Can I ask, if should the Icw of the Switchgear be the same as the Icu of the Circuit Breaker.
For example I will use Icw=85kA for the LV Switchgear, should I use 85kA for each protective devices inside that LV Switchgear?
A. 1. For "Switchgear=ASSEMBLY"
Reference IEC 61439-x "Rated short-time withstand current (Icw) of a circuit of an ASSEMBLY is the value of short-time current...which that circuit can carry without damage under the test condition.... Unless otherwise stated...the time is 1s".
A complete Icw rating should comprises of the [current value] and the [time duration] e.g. 85kA 1s or 50kA 3s etc. If the duration is not stated, it is taken as 1s.
It is an assessment of the [busbar size, spacing], [busbar holder span and mechanical strength] and the [mechanical structure of the ASSEMBLY].
2. For "protective devices=Circuit breakers"
Reference IEC 60947-x "Rated ultimate short-circuit breaking capacity (Icu)...is the breaking capacity for which ... do not include the capability of the circuit-breaker to carry its rated current continuously"
The Icu rating comprises of the [voltage value] and the [current value] e.g. 50-60Hz 380/415V 70kA or at 690V 40kA etc.
3. All circuit breakers including the incoming and the outgoings shall be selected based on the short-circuit current level. The ASSEMBLY may be equal to or higher than the short-circuit current level.

Che Kuan Yau (Singapore)

 

[RRaghunath]

There is one more parameter - Ics. This is the fault level the circuit breaker can interrupt without suffering any damage. So, Ics shall be more than the rated SC fault level of the switchboard.
High current circuit breakers have Ics equal to Icu so no problem. It is the low rated CBs and MCCBs one needs to be careful while selecting.

 

[che12345]

Dear MrRRaghunath

1. " There is one more parameter - Ics. This is the fault level the circuit breaker can interrupt without suffering any damage".
Reference IEC 60947-x Rated service short-circuit breaking capacity (Ics) .... to a [specified test sequence]* including the capability of the circuit-breaker to carry its rated current continuously.
My observations in the testing station immediately after the ICS test:
The breaker does show heavy damages in the contacts, arc-chute plates and removable contacts etc. However, the breaker subsequently goes through [specified test sequence]* tests mechanically (drawing in/out 100 cycles), creepage+clearance distances measurements, electrical insulation, dielectric, impulse and temperature-rise tests etc., to ascertain that the capability of the circuit-breaker is capable to carry its rated current continuously.

2. " So, Ics shall be more than the rated SC fault level of the switchboard".
a) If the incoming is an ACB, its Ics is likely to be equal or higher than the short-circuit current level.
b) If the incoming and feeder breakers are MCCBs, they are likely to be selected based on Icu, which shall be equal or higher than the short-circuit current level.
c) The switchboard=ASSEMBLY shall be equal or higher then the short-circuit current level.

3. " High current circuit breakers have Ics equal to Icu so no problem. It is the low rated CBs and MCCBs one needs to be careful while selecting".
In general:
a) For ACBs : Ics=Icu
b) For MCCBs : Ics = 50%, 75& or 100%Icu

Che Kuan Yau (Singapore)