Answer Posted / h.shastri

almost all answer is somewhat correct

the main thing is to selection of proper MCB which matches
with your equipments and instruments

for that i'd like to give you an case study...

let's Assume one Heating Control Action

suppose i am using a 1ph. heater of 1200 Watt (5.2 Amps)
now i am controlling heating action using an SSR or
relay/contactor

now if i will select SSR (relay/contactor)'s rating =6 amps
and i also impliment MCB of 6 Amps.b-curve

then in case of heater failer or implimenting different
rating's heater,

assume that circuit is taking some higher current say, 10
Amps in such my SSR(relay/contactor)will damage due to high
load current which is higher then their rating of 6 Amps

but MCB will not trip instently ( it trips after 5-10 min.
because load current is not more then 12 Amps)

in this case MCB will not protect your instruments.

now if i'll decrease MCB's rating it will starts tripping
on and on.....

if i'll select MCB more then 6 Amps again in this case MCB
will not trip

so that, if i m using b curve 6 Amps MCB then i must select
SSR(relay/contactor)'s rating more then 12 Amps say 15Amps.
then and then problem will be solve.

also if i will use MCB of C or D curve then Again i have to
choose instruments which has atleast 3-5 times higher
rating of MCB.

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