Is It Safe to Use One MCB for Multiple 3-Phase DOL Starters With Individual Overloads?

Hi, Is there anyone who can offer any advice on motor  starter design?
Assuming that adequately sized thermal overloads/contactors are specified; is there any reason why it is not possible to use a single shared MCB for line  power distribution to multiple DOL starters (AC-3P). Please see attached (crude) sketch.
My thinking is that the  O/L relay would provide circuit protection as well as preventing motor burnout??
any advice would be greatly appreciated
Thanks

Hi Andrew-The thermal overloads are intended mainly to trip if the motor is overloaded. I don't think they would be fast enough to give good protection in the even of a short circuit or catastrophic failure.I think your scheme would work **most** of the time, but I would not want to risk it.  I think it is still prudent to have a properly sized breaker for each motor.-Rick  

Typically, the contactor will be controlled by separate start and stop buttons, and an auxiliary contact on the contactor is used, across the start button, as a hold in contact. I.e. the contactor is electrically latched closed while the motor is operating.
Principle of Direct On Line Starter (DOL)To start, the contactor is closed, applying full line voltage to the motor windings. The motor will draw a very high inrush current for a very short time, the magnetic field in the iron, and then the current will be limited to the Locked Rotor Current of the motor. The motor will develop Locked Rotor Torque and begin to accelerate towards full speed.
As the motor accelerates, the current will begin to drop, but will not drop significantly until the motor is at a high speed, typically about 85% of synchronous speed. The actual starting current curve is a function of the motor design, and the terminal voltage, and is totally independent of the motor load.
The motor load will affect the time taken for the motor to accelerate to full speed and therefore the duration of the high starting current, but not the magnitude of the starting current.
Provided the torque developed by the motor exceeds the load torque at all speeds during the start cycle, the motor will reach full speed. If the torque delivered by the motor is less than the torque of the load at any speed during the start cycle, the motor will stops accelerating. If the starting torque with a DOL starter is insufficient for the load, the motor must be replaced with a motor which can develop a higher starting torque.
The acceleration torque is the torque developed by the motor minus the load torque, and will change as the motor accelerates due to the motor speed torque curve and the load speed torque curve. The start time is dependent on the acceleration torque and the load inertia.
DOL starting have a maximum start current and maximum start torque.This may cause an electrical problem with the supply, or it may cause a mechanical problem with the driven load. So this will be inconvenient for the users of the supply line, always experience a voltage drop when starting a motor. But if this motor is not a high power one it does not affect much.
Parts of DOL StartersContactors & CoilDOL part -ContactorDOL part -ContactorMagnetic contactors are electromagnetically operated switches that provide a safe and convenient means for connecting and interrupting branch circuits.
Magnetic motor controllers use electromagnetic energy for closing switches. The electromagnet consists of a coil of wire placed on an iron core. When a current flow through the coil, the iron of the magnet becomes magnetized, attracting an iron bar called the armature. An interruption of the current flow through the coil of wire causes the armature to drop out due to the presence of an air gap in the magnetic circuit.
Line-voltage magnetic motor starters are electromechanical devices that provide a safe, convenient, and economical means of starting and stopping motors, and have the advantage of being controlled remotely. The great bulk of motor controllers sold are of this type.
Contactors are mainly used to control machinery which uses electric motors. It consists of a coil which connects to a voltage source. Very often for Single phase Motors, 230V coils are used and for three phase motors, 415V coils are used. The contactor has three main NO contacts and lesser power rated contacts named as Auxiliary Contacts [NO and NC] used for the control circuit. A contact is conducting metal parts which completes or interrupt an electrical circuit.
NO-normally openNC-normally closed
 Over Load Relay (Overload protection)Overload protection for an electric motor is necessary to prevent burnout and to ensure maximum operating life.
Under any condition of overload, a motor draws excessive current that causes overheating. Since motor winding insulation deteriorates due to overheating, there are established limits on motor operating temperatures to protect a motor from overheating. Overload relays are employed on a motor control to limit the amount of current drawn.
The overload relay does not provide short circuit protection. This is the function of over current protective equipment like fuses and circuit breakers, generally located in the disconnecting switch enclosure.The ideal and easiest way for overload protection for a motor is an element with current-sensing properties very similar to the heating curve of the motor which would act to open the motor circuit when full-load current is exceeded. The operation of the protective device should be such that the motor is allowed to carry harmless over-loads but is quickly removed from the line when an overload has persisted too long.
Normally fuses are not designed to provide overload protection. Fuse is protecting against short circuits (over current protection). Motors draw a high inrush current when starting and conventional fuses have no way of distinguishing between this temporary and harmless inrush current and a damaging overload. Selection of Fuse is depend on motor full-load current, would “blow” every time the motor is started. On the other hand, if a fuse were chosen large enough to pass the starting or inrush current, it would not protect the motor against small, harmful overloads that might occur later.
The overload relay is the heart of motor protection. It has inverse-trip-time characteristics, permitting it to hold in during the accelerating period (when inrush current is drawn), yet providing protection on small overloads above the full-load current when the motor is running. Overload relays are renewable and can withstand repeated trip and reset cycles without need of replacement. Overload relays cannot, however, take the place of over current protection equipment.
The overload relay consists of a current-sensing unit connected in the line to the motor, plus a mechanism, actuated by the sensing unit, which serves, directly or indirectly, to break the circuit.

Regularly, the contactor will be controlled by isolated begin and stop catches, and an assistant contact on the contactor is utilized, over the begin catch, as a hold in contact. I.e. the contactor is electrically hooked shut while the engine is working.
Rule of Direct On Line Starter (DOL)
To begin, the contactor is shut, applying full line voltage to the engine windings. The engine will draw a high inrush current for a brief span, the attractive field in the iron, and after that the present will be restricted to the Bolted Rotor Current of the engine. The engine will create Bolted Rotor Torque and start to quicken towards full speed.
As the engine quickens, the present will start to drop, yet won't drop altogether until the point that the engine is at a rapid, ordinarily about 85% of synchronous speed. The genuine beginning current bend is an element of the engine plan, and the terminal voltage, and is absolutely free of the engine stack.
The engine load will influence the time taken for the engine to quicken to full speed and in this manner the term of the high beginning current, yet not the greatness of the beginning current.
Given the torque created by the engine surpasses the heap torque at all velocities amid the begin cycle, the engine will achieve full speed. In the event that the torque conveyed by the engine is not exactly the torque of the heap at any speed amid the begin cycle, the engine will quits quickening. In the event that the beginning torque with a DOL starter is lacking for the heap, the engine must be supplanted with an engine which can build up a higher beginning torque.
The quickening torque is the torque created by the engine short the heap torque, and will change as the engine quickens because of the engine speed torque bend and the heap speed torque bend. The begin time is subject to the speeding up torque and the heap inactivity.
DOL beginning have a most extreme begin present and greatest begin torque.
This may cause an electrical issue with the supply, or it might cause a mechanical issue with the determined load. So this will be badly arranged for the clients of the supply line, dependably encounter a voltage drop when beginning an engine. In any case, if this engine is anything but a powerful one it doesn't influence much.
Parts of DOL Starters
Contactors and Loop
DOL part - Contactor
DOL part - Contactor
Attractive contactors are electromagnetically worked switches that give a protected and advantageous means for associating and interfering with branch circuits.
Attractive engine controllers utilize electromagnetic vitality for shutting switches. The electromagnet comprises of a loop of wire put on an iron center. At the point when a present course through the curl, the iron of the magnet ends up polarized, pulling in an iron bar called the armature. An intrusion of the present move through the curl of wire makes the armature drop out because of the nearness of an air hole in the attractive circuit.
Line-voltage attractive engine starters are electromechanical gadgets that give a protected, helpful, and practical methods for beginning and halting engines, and have the upside of being controlled remotely. The extraordinary greater part of engine controllers sold are of this sort.
Contactors are fundamentally used to control apparatus which utilizes electric engines. It comprises of a curl which associates with a voltage source. All the time for Single stage Engines, 230V curls are utilized and for three stage engines, 415V loops are utilized. The contactor has three fundamental NO contacts and lesser power evaluated contacts named as Assistant Contacts [NO and NC] utilized for the control circuit. A contact is leading metal parts which finishes or intrude on an electrical circuit.
NO-typically open
NC-typically shut
Over Load Transfer (Over-burden insurance)
Over-burden insurance for an electric engine is important to avert burnout and to guarantee most extreme working life.
Under any state of over-burden, an engine draws unnecessary current that causes overheating. Since engine winding protection disintegrates because of overheating, there are built up points of confinement on engine working temperatures to shield an engine from overheating. Over-burden transfers are utilized on an engine control to restrict the measure of current drawn.
The over-burden transfer does not give cut off. This is the capacity of over current defensive hardware like wires and circuit breakers, by and large situated in the disengaging switch fenced in area.
The perfect and most straightforward route for over-burden security for an engine is a component with current-detecting properties fundamentally the same as the warming bend of the engine which would act to open the engine circuit when full-stack current is surpassed. The task of the defensive gadget ought to be with the end goal that the engine is permitted to convey safe over-loads however is immediately expelled from the line when an over-burden has persevered too long.
Regularly melds are not intended to give over-burden security. Wire is ensuring against shortcircuits (over current security). Engines draw a high inrush current when beginning and customary circuits have no chance to get of recognizing this transitory and safe inrush current and a harming over-burden. Choice of Wire is rely upon engine full-stack current, would "blow" each time the engine is begun. Then again, if a breaker were picked sufficiently extensive to pass the beginning or inrush current, it would not ensure the engine against little, destructive over-burdens that may happen later.
The over-burden transfer is the core of engine insurance. It has opposite outing time qualities, allowing it to hold in amid the quickening time frame (when inrush current is drawn), yet giving insurance on little over-burdens over the full-stack current when the engine is running. Over-burden transfers are sustainable and can withstand rehashed excursion and reset cycles without need of substitution. Over-burden transfers can't, be that as it may, replace over current security hardware.
The over-burden hand-off comprises of a current-detecting unit associated in the line to the engine, in addition to a system, impelled by the detecting unit, which serves, straightforwardly or in a roundabout way, to break the circuit.