Type 2 Coordination
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Type 2 Co-ordination in Low Voltage Motor Starters Nowadays, any discussion on Low Voltage Switchgear is not complete without mentioning the words Type 2 Co-ordination at least once. What is this Type 2 Co-ordination? First of all, what is co-ordination? Co-ordination is nothing but the ability to perform well as a team. Brilliant performances individually may not mean that they would meet with similar success in a team as well. For example, a sprinter might win gold medal in 100 meters individual dash event. But, it is not guaranteed that a 4-x100 meter relay race team with this sprinter as one of its members would also stand out in the race. Because the sprinter might be brilliant in the individual race but when it comes to performance in a team, the same sprinter might fail miserably. What counts here is performance in and as a team and not individual performances. Similarly, in a power system too, rather than the individual performances, the performance as a team in the network is what counts when it comes to meriting the performance of switchgear & controlgear. Usually, the selection of a switchgear and/or control gear component for a motor starter feeder is done, based on the kW or HP rating of the motor and in rare cases, also based on the utilization category of the equipment. But, this may not suffice. Consideration must also be given to the fact that how the constituent components of the starter/power system switchgear/controlgear would work in all conditions of normality and abnormality. The study of such considerations is briefly termed as co-ordination. The ill effects of poor co-ordination could be:
Blowing of Fuses and/or tripping of Circuit Breaker during Motor Starting Heavy arcing inside the contactors, leading to welding of contacts Distortion of Bimetal Overload Relay Elements, which may fail to respond to overloads Repair/Replacement costs would be very high Loss of production due to increased downtime
We all know well that an induction motor, when started direct on line, would draw about 6 to 8 times its rated current as the starting current. But, what we do not realize is that this starting current itself consists of two parts (viz.) the transient part and the steady state part. The transient part of the staring current is of the magnitude of about 12 times the rated current of the motor and might extend for a period of about 10 milli seconds. This is due to the magnetic inrush phenomenon and hence this transient part of starting current can also be called as “inrush current”. After about 10 milliseconds, the starting current stabilizes to its steady state value between 6 and 8 times the rated motor current and subsidizes further after the expected starting time, which is dependant upon the inertia of the drive and many other parameters. (Refer Figure 1).
The magnetic inrush current of a motor could be viewed as short circuit by an underrated fuse and the fuse would blow. Also, wrongly set instantaneous release of an upstream circuit breaker could trip. In such conditions one can never be able to start a motor. Isn’t it? FIGURE 1 - TYPICAL CURVE OF CURRENT DURING A D.O.L. START CURRE CURRENT
Similarly, even relatively low levels of fault currents would produce electromagnetic forces that would cause the contacts of a contactor to “lift-off”, due to contact arrangement based on the repulsion principle. The arcing caused due to the fault current in this situation would cause the contacts to weld together. Also, the let-through energy level, if exceeds the withstand I2t limits of the contactor or the overload relay, might cause melting of contacts and contact welding in contactors and distortion of the bimetal element in overload relays. Such distortion of the bimetal element could prevent restoration of the element to its original shape and position upon cooling, thereby destroying the protection properties of the overload relay itself. Worst part is that such damages are not known immediately and might lead to spurious/nuisance trippings or motor failures – for, the relay may not operate when needed to or might operate for healthy conditions too. As we all know, a motor starter is a combination of an SFU or SDF Unit, a contactor and an overload relay. The overload relay protects the motor from sustained overloads during the normal running of the motor. The time of operation of the overload relay is inversely proportional to the current flowing.
But, during short circuits, the response time of the overload relay might be too long to offer sufficient protection to the constituent components of the circuit to be protected. In this case the fuse or any other short circuit protective device (SCPD) should take over to interrupt the fault current and also to limit the let through energy to the starter components. Also, the IS Specification clearly defines the making & breaking capacity limits for contactors under different utilization categories (Refer Table 1.0). Table 1.0 Making & Breaking Capacities of Contactors (As per IS 13947 Part 4 Section 1) Make & Break Conditions Ic/Ie Ur/Ue Cos θ 1.5 1.05 0.8 4.0 1.05 0.65 8.0 1.05 0.35 10.0 1.05 0.35 Make Conditions 10.0 1.05 0.35 12.0 1.05 0.35
Utilisation Category AC 1 AC 2 AC 3 AC 4 AC 3 AC 4
It should not so happen that the overload relay clears the faults beyond the breaking capacity of the contactor, thereby damaging the contactor. However, if the overload relay fails to clear the fault, then the fuse or the SCPD shall take over and clear the overload fault albeit with a longer time delay, but well within the let-through energy capabilities of the contactor and the overload relay. It should also be seen that for all fault currents within the breaking capacity limit of the contactor, only the overload relay should clear the fault and not the fuse or the SCPD, for, if the fuse or SCPD clears such low level faults, it involves unnecessary expenditure in replacement of fuses and also it would reduce the life expectancy of the SCPDs. Also, the operating time of OLR and/or SCPD shall be such that the I 2t limits of the contactor and the overload relay are not exceeded. (Refer Figure 2) FIGURE 2 – TYPE 2 CO-ORDINATION CURVES
T I M E RELAY
CURRENT (n × IrM )
If such a combination could be achieved in a motor starter feeder then the feeder is said to be perfectly co-ordinated. The IEC as well as IS recognize two types of Co-ordination (viz.) Type 1 & Type 2. Type 1 Co-ordination requires that under short circuit conditions, the contactor or starter shall cause no danger to persons or installation and may not be suitable for further service without repair and replacement of parts. Type 2 Co-ordination requires that under short circuit conditions, the contactor or starter shall cause no danger to persons or installation and shall be suitable for further use. The risk of contact welding is recognized, in which case the manufacturer shall indicate the measures to be taken as regards the maintenance of the equipment. Type 1 Co-ordination may not prevent damage to the motor starter components. In order to ensure that high-level fault currents do not interrupt a critical process, Type 2 Co-ordination should be used in the selection and application of components for low voltage motor starters. Type 2 Co-ordination does not permit damage to the starter components beyond light contact welding in the contactor, which could easily be separated by a screwdriver. Type 2 Co-ordination does not permit the replacement of parts (except fuses) and requires that all parts remain in service. From the above, it is obvious that having Type 2 Co-ordination is better, for, even after clearing a short circuit, the starter is ready for re-use almost immediately, without the need to repair or replace any parts – except perhaps, the fuses. We can establish this through a case study. Consider a 3phase, 415V, AC Squirrel Cage Induction Motor of Rating 50HP. The full load current is 70A and it is assumed that the motor is started Direct-on-Line. The locked rotor current is assumed to be 6 times the rated motor current and the starting time is 4 seconds. The utilization category is AC 3. Now let us select switchgear for this application. The AC 3 nominal current rating of the contactor should be greater than 70A; considering the rated making capacity and rated breaking capacity requirements, we can choose an 80A Contactor for this application. The overload relay selected should be of the range 45-75A and the relay should be set at 70A. For HRC Fuse selection, first the nominal current rating of the fuse shall be greater than the motor full load current of 70A. The nearest rating fuse available is 80 A fuse. Supposing we select an 80A Fuse, it would blow in about 180 milli seconds for the inrush current magnitude of 12 x 70 A (i.e.) 840A. Hence, it would not blow for the inrush current, whose duration is only 10 milliseconds. Next, for the starting current magnitude of 6 x 70A (i.e.) 420A, this fuse would blow in about 3 seconds. So, this is not suitable, as the starting time of the motor is 4 seconds. We have to go for the next higher size of fuse available (i.e.) 100A. The 100A use would blow in about 210 milli seconds for the inrush current and in about 10 seconds for the starting current. So, this is suitable for this motor both for starting current as well as the inrush current.
We have found out that for the above motor an 80A Contactor, a 45-75A OLR and a 100A Fuse would be OK, if selected individually. Now, let us see, how they would perform in co-ordination with each other. For this let us construct a table comparing the tripping times of the OLR and two ratings of fuses for various multiples of the rated motor current: Current ► OLPD/SCPD▼ OLR (set at 70A) 100A Fuse 125A Fuse
5 Irm (350A) 12 18 50
6 Irm (420A) 9 10 20
7 Irm (490A) 7.5 5 12
8 Irm (560A) 6.5 9.0
9 Irm 10 Irm (630A) (700A) 5.5 5.0 Not Required 5.0 Not Reqd.
As can be seen from the above table, even though the 100A Fuse was found to be suitable for this motor in view of inrush current & starting current withstand capabilities, this 100A Fuse is not suitable if viewed in co-ordination with the contactor & relay. Because, even for a current of 490A, which is only about 6 times the contactor rating and which, the contactor can satisfactorily break, the fuse takes over and blows out. This means, for faults, which could be easily cleared by the OLR & Contactor, the fuse would blow, thereby incurring expenditure on replacement of fuses. Hence, even as this fuse was found suitable for this application if seen in isolation, the same fuse is not suitable if co-ordination studies are taken up. Here, a 125 A Fuse serves the purpose, both in isolation as well as in co-ordination, for, this fuse allows the OLR to clear faults up to 560A (about 7 times the contactor rating) and takes over at 630 A, which is about 8 times the contactor rating. This suits the purpose even in co-ordination requirements. Similar studies can be undertaken for Star-Delta started motors too. Reputed switchgear/Controlgear manufacturers provide selection charts for selecting components for motor feeders for achieving Type 2 Co-ordination. Charts are available for both DOL Starter feeders and for Star-Delta Starter feeders. Also, charts are available for SDF fed feeders as well as for MCCB/MPCB fed units for both the above types of starters. It is in the interest of the Electrical Professionals to refer to such charts, before undertaking any motor feeder design. One thing must be remembered: “IT IS EASY TO ESTABLISH TYPE 2 CO-ORDINATION, BUT VERY DIFFICULT TO MAINTAIN IT.” Because, when a new panel is ordered, one could ask the panel builder to provide starter feeder components as per Type 2 Co-ordination requirements. But, once, when the panel is in service and upon a fuse blow up in the motor feeder, the tendency of the operating person is to provide the replacement with whatever rating fuse available in hand to him at that time. If the so replaced fuse is of much over rating, then even Type 1 Co-ordination is lost and it might become fatal too, if the fault persists. Hence, care must be taken to ensure that even breakdown replacements of starter feeder components adhere to Type 2 Co-ordination requirements.