Electronic Trips in MCCB

[charz]

There are LSIG, LSI, LS, LI type electronic trip units in MCCB. I understand that it provides Long-time, Short-time, Instantaneous, Ground fault type of protection depending on the type of electronic trip unit. One would normally prefer LSIG type electronics trip unit as it offers all the protection in one unit.
LI: Why would one do not want to use short time and ground fault protection in his application?
LS: Why would one do not want to use Instantaneous and ground fault protection in his application?
LSI: Why would one do not want to use ground fault protection in his application?
In short, I would like to know, In what type of applications the LSI, LS, LI type of electronic trips are used?

 

[davidbeach]

The G won't do anything useful on an impedance grounded system, so that's the most likely reason to omit G.

LI may be sufficient if all the breaker is feeding are motors where the S would be provided by the motor starters.

LS would be used where the presence of the I produces coordination problems. In many cases there is an I at a high level that can't be adjusted.

 

[jraef]

If replacing a basic Thermal-magnetic MCCB, that was the equivalent of LI trips anyway, so adding an S might actually make things more complicated.
I don't think I have ever seen an LS trip unit; LI, LSI, or LSIG


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[ppedUK]

It depends what type of MCCB you have. For instance, in IEC standards, there are Category 'A' and Category 'B' MCCBs. Category 'A' MCCBs do not have a withstand rating and therefore can usually only be fitted with LI releases, as you are not permitted to have a time delay under short circuit condition (as you would when using an 'S' element). These would normally be used to protect a feeder to an item of plant (e.g. a compressor package). Category 'B' MCCBs can be fitted with LSI (or LSIG) releases with an intentional time delay (S) and these would normally be fitted to feeders to other distribution assemblies (e.g. panelboards or fuseboards). Just to confuse matters further, there is at least one manufacturer that offers an LSoI release for a category 'A' MCCB, but the 'S' element has fixed settings which ensure a trip within the device's withstand capability (though it is not a published withstand capability). MCCBs by the way, have very much lower withstand capability than open construction ACBs.

The catalogue may show all available protection modules as if you can choose to fit any option to your chosen MCCB, but that may not be the case.

With regard to the choice of a 'G' element, this is usually a definite time characteristic only (though some have I^t cropping, but this is only a very small slope) and almost always causes a problem grading with downstream fuses. Quite often the 'G' element cannot be switched off, so you are then stuck with it if you standardise on all modules being LSIG.

Some manufacturers provide releases which mimic more conventional IDMT curves and this may solve the grading issue with the 'G' element.

LS is usually not an option.