Frequently Asked Questions (MCCBs)

Frequently Asked Questions(MCCBs)

Q1.What are the advantages of MCCBs as a back-up protective device?

MCCBs offer the following advantages :

MCCBs are Compact.They save considerable panel space
MCCBs minimize dowmtime.Unlike in a fuse-based system,there's no searching for a replacement fuse.MCCBs can be Reset & Switched On immediately after clearing the fault that caused the tripping.
MCCBs minimize inventory.Unlike fuses, they are not "consumables" and hence there is no need to stock MCCBs the way fuses have been stocked.An MCCB can clear several faults before it is due for replacement.
MCCBs are "Maintenance free" and hence the recurring costs are minimal.
There is no possiblity of single phasing due to fault in only one phase when MCCBs are used.
Multi-purpose accessories can be fitted with MCCBs.

Q2.Do MCCBs have short time withstand rating?
MCCBs are downstream protective devices, and hence are expected to sense & clear circuit faults instantaneously without any delay. For this reason, MCCBs do not need to have any shortime withstand rating. However, MCCB's of ratings 1000A & above are also available with short time withstand rating.

Q3.What is utilization category & what category is assigned to MCCBs?

Switchgear standard IS13947 (part2) / IEC 947-2 has defined two types of utilization category type "A" & type "B".Circuit beakers which are intended to be used for selective tripping (i.e offering time dicrimination with downstream devices) are assigned utilization category "B".Air Circuit breakers(ACBs) fall in this category.Products with utilization category "B" must have a short time withstand rating (Icw).

Q4.Are MCCBs suitable for DC applications?
Yes, MCCBs with thermal-magnetic releases are suitable for DC application. The thermal rating in these MCCBs remain unchanged.However MCCBs with static or microprocessor based release are not suitable for DC applications.

Q5.What types of protection is offered by MCCBs in an installation?
Standard MCCBs offer overload and short circuit protection to an installation. However, accessories are available to enable an MCCB to offer earth fault, earth leakage and undervoltage protection as well.

Q6.How many short circuit faults can an MCCB clear?
As per standard IS 13947 (part2) / IEC 947-2, an MCCB can clear following nos. of short circuit faults: At fault current = Icu(claimed),2 nos. of S.C. faults At fault current = Ics(claimed),3 nos. of S.C. faults

Q7.When should an MCCB be replaced,i.e after clearing how many short circuit faults?

When a MCCB clears short circuit faults,it results in

Weakening of dielectric strength.In case dielectric strength falls to a low value,it may lead to flashover during subsequent usage.It is therefore advisable to measure the megohm resistance between phases & across open contacts of the MCCBs.If the value falls below 1 Megaohm (even after cleaning), it is advisible to replace the MCCB.
Erosion of contact buttons.This results in higher contact resistance and subsequent overheating/nuisance tripping of the MCCBs. To evaluate the extent of contact erosion,millivolt drop should be measured across the contacts/terminals.In case these values exceed the limit suggested by the manufaturer, the MCCB needs to be replaced.

Q8.What are the applications in which MCCBs can be used?
MCCBs can be used in wide ranging applications.Most common usage of MCCBs are in the following applications: Distribution feeders. Transformer feeders. Generators feeders. Motor feeders. UPS/Battery feeders. Furnace feeders. DC load feeders.

Larsen & Toubro - Electrical Business Group