Overload Protection with DOL Starter

The MCB should be rated at up to 250% of motor FLC. The MCB provides short circuit protection. Overload protection is provided separately, based on the nameplate FLC of the motor.

Read THIS↓carefully, twice, then you won't have to ask again.

Motor Calculations - Electrical Construction & Maintenance

Thanks Iyn for your attached link. If follow the NEC rules, sizing the overload protection for motor with DOL starter would be mainly based on the FLA. That means we totally no need to bother about the high starting current at overload protection MCB because all these have been tackle by the DOL starter, am I right ? I understand that DOL starter is only built with overload relay, circuit breaker etc. What is the principle behind which made a DOL starter can develop high current with the low inline current during start up. Please correct me.

"Overload protection MCB" is an incorrect phrase; the circuit breaker is not considered as overload protection.

Inverse-time CBs can carry progressively larger currents for progressively smaller lengths of time. I.e., a CB at 250% FLA can support starting current of say 6x FLA for several seconds, as can be seen from its trip curve.

What standards apply?

The fact that you have no idea if this is OK, or not, and don't know what standards apply leads me to believe that I should simply say, hire some competent help, fast.

<unsubscribes rapidly>

Someone should have read the country and site standards and codes of practice before installing the earthing system.

If this is your job to see if this is correct or not you will need to obtain and read the standards yourself. If this is general interest, I don't have access to your country and site standards and codes of practice.

You seem to have accidantally replied to the wrong thread. There was a recent (5/23) thread titled "Grounding" that might be helpful.

Why does it always start with "I have doubt?"

O/L's set to FLC of the motor.

MCCB's set to protect the installation with allowance for the inrush a n d ……….

I'm in a bad mood!

"Why does it always start with "I have doubt?""

Tony, after many years of following this forum, I came to the realization that non-English speakers are mis-translating this from what we would say as "I have a question"

check this

http://www.zone4info.com/search?query=Overload+Protection+with+DOL+Starter

MCBs MUST not be used for motor overload protection. The thermal overload protection charactersitics prescribed for MCBs (IEC 898 Curve) are for the protection of cables and NOT for motors. Motors need to follow the IEC 647 Curves for overload protection.

However, MCBs can be used as a back-up switching device cum short circuit protective device - if properly selected.

If your problem is MCB tripping due to the high inrush current and/or the starting current of the motors, then this may be easily overcome by selecting a "D" curve MCB for the purpose.

Pl. learn a bit about the B, C & D Curves of MCBs.

I firmly agree that the demonstrated lack of understanding of basic motor protection and control concepts means that the OP is unqualified to be performing this work.

But for the benefit of the others in our community who can likely argue this ad nauseum, I perceive a possible semantics issue here that the OP may be fostering BECAUSE of his lack of understanding AND use of acronyms.

MCB in North America means Main Circuit Breaker

MCB in most of the English speaking world means Miniature Circuit Breaker

MCB in many non-English speaking countries is often mistakenly used to mean Motor Circuit Breaker, referring to the devices that are a combination of CB and adjustable overload relay otherwise known by many other names, depending on the manufacturer, such as Motor Circuit Protector (which also has a different meaning in NA), Motor Protective Switch, Motor Switch Protector etc.

The <...misconceptual...> is that of not using the applicable national standard for such things, such as British Standard 7671.

One cannot protect a motor against overload with an MCB for that device is there to protect the cabling downstream. The correct device to protect the motor is a motor overload. The technique needed to get these two to work together in the same circuit is called "co-ordination", the detail of which is all in BS7671.

Refer literature from MCB manufacturers like Hager,ABB,Siemens,Schneider/Merlin Gerin etc