Momentary Duty and Interrupting Duty
Momentary Duty and Interrupting Duty
(OP)
I have a 2MVA 11kV/400V Transformer, Secondary is connected to a busbar through a MCCB.
ETAP calculations gave following report on duty checking.
1. Momentary Duty--------
Sym KA rms = 52.063
x/R = 2.7
mf = 1.095
Asymmm kA sym= 96.877
2 Interrupting Duty-------
Sym KA rms = 52.063
x/R = 2.7
mf = 1.0
Adj sym kA rms= 52.063
I have selected MCCD Rated Int 65, Test pf =20, 65kA based upon the Interrupting Duty I got above.
My question is what is the purpose of Momentary Duty (Asymmm kA sym= 96.877) and is it taken care of when we select a breaker.
ETAP calculations gave following report on duty checking.
1. Momentary Duty--------
Sym KA rms = 52.063
x/R = 2.7
mf = 1.095
Asymmm kA sym= 96.877
2 Interrupting Duty-------
Sym KA rms = 52.063
x/R = 2.7
mf = 1.0
Adj sym kA rms= 52.063
I have selected MCCD Rated Int 65, Test pf =20, 65kA based upon the Interrupting Duty I got above.
My question is what is the purpose of Momentary Duty (Asymmm kA sym= 96.877) and is it taken care of when we select a breaker.






RE: Momentary Duty and Interrupting Duty
Asymm current depends on the X/R ratio so it is not the same for every system of a given symmetrical currents. While the breaker are tested to a given ratio per ANSI standards, you still need to check that the calculated asymm (momentary) duty is still within capability of the device. You can also make the adjustments based on the actual X/R ratio.
Rafiq Bulsara
http://www.srengineersct.com
RE: Momentary Duty and Interrupting Duty
So do you think in this case as Asymmm kA sym= 96.877 I must go for ACB?
RE: Momentary Duty and Interrupting Duty
If available, try a higher rated MCCB such as 85kA or 100kA rms symmetrical would work. The gear also need to be rated accordingly.
Check the momentary duty report in your software. It should also show rated Momentary duty vs calculated.
Personally, I would prefer a ICCB or ACB for a secondary main of 2MVA substation. Provides better coordination and more rugged.
Rafiq Bulsara
http://www.srengineersct.com
RE: Momentary Duty and Interrupting Duty
The momentary duty is accounted for in the 65kA symmetrical rating for X/R up to a certain level (4.9 for MCCB's in the ANSI world). The Interrupting Duty calculation appears to be showing you that no adjustment is needed due to the X/R of your system (2.7).
I'm not an ETAP person and this is my guess as to what it's saying. Read the software manual carefully to be sure.
Alan
"The engineer's first problem in any design situation is to discover what the problem really is." Unk.