BACKGROUND
Solid state switching of AC current has been a practical proposition since the 1950‘s when the Thyristor or Silicon Controlled Rectifier (SCR) and it‘s cousin the Triac were developed.
SCRs and Triacs are the basic switching unit used in Solid State Relays and Phase Angle or Burst Power Controllers. When used correctly this type of switch can operate reliably over millions of cycles. Other uses for SCRs and Diodes include heating control, rectifiers and motor drives.
Any semiconductor device may be damaged by over current, SCR’s, Diodes and Triacs are no different. Fortunately, they are quite robust devices and may be easily protected by the use of a high speed "semi-conductor" fuse.
SEMICONDUCTOR FUSING
One of the most misunderstood areas of semiconductor switch application is fusing. Fuses designed for semiconductor protection are extremely fast acting and are selected for their current rating as well as for their I2t (or let through energy) rating. Conventional switchboard circuit breakers and HRC fuses are too slow to provide any protection to semiconductor devices.
Semiconductor fuses are designed to protect semiconductors only. Given the right environmental conditions, they can survive for long periods above their nominal current rating allowing possible damage to wiring and insulation. Therefore conventional HRC fuses of circuit breakers should be installed as well for this purpose.
Some Semiconductor fuses sold in Australia appear to have no standards approvals. High quality fuses that are tested to international standards such as BS88 Part 4, VDE0636 Part 23, or IEC269-4 should always be used.
PERMITTED OVERLOADS
In many applications surges occur during the normal operation of equipment, and under such permitted overloads the fuse must not operate. For overloads lasting longer than a few minutes, the fuse should be selected on the basis of the continuous current rating of the fuse being greater than the overload current.
For infrequent normal overloads, fuse selection can be made on the basis of an overload curve (see Figure 2) and can be taken as 75% of the published time – current curve.
For repetitive overloads, such as a motor drive or soft starter, fuse selection can be made on the basis of an overload curve (see Figure 2) and can be taken as not more than 60% of the published time – current curve.
CURRENT RATING
The maximum current carrying capability is at least equal to the current rating of the fuse provided that the ambient temperature is less than 30 deg C, and conductor sizes are at least 1 mm2 per Amp and there is natural convection cooling.
For ambient temperatures higher than 30 deg C, a de-rating of 0.5% per deg C is recommended. In situations where the air temperature around the fuse is higher than ambient, e.g. inside a converter cubicle, the de-rating should be based on the actual temperature of the air surrounding the fuse.
In many installations power semiconductors are fan cooled. If the fuses are mounted in the air flow, their maximum current carrying capability can be increased by 20% in an air flow of 5m/s. Higher airflows will not produce any further increase in rating.
Semiconductor fuses dissipate heat and are designed to run "warm". They should never be enclosed in cartridge type fuse holders as this restricts heat dissipation causing the fuse to run hot and "blow" at a lower point than expected.
I2t CHARACTERISTICS
The pre arcing or melting I2t of semiconductor fuses is independent of voltage and reduces to a minimum value for times less than about 1ms. For longer times the time – current curve (see Figure 2) should be used.
A fuse should be chosen such that the I2t of the fuse is less than the I2t of the semiconductor device it is protecting. A margin of 25% is normally used.
Typical I2t values for operating times less than one mains half cycle are normally shown in data sheets. They normally relate to fault currents of 20 x rated current, and a power factor of 0.2.
The I2t values reduce considerably at operating voltages below rated voltage, as the interruption of the arc inside the fuse, while it operates, is easier.
Typical values of arc voltages, related to operating voltage Un of fuses are:
Operating Supply Voltage Arc voltage of fuse
Un 2.0 Un
0.5 Un 1.25 Un
0.2 Un 0.7 Un
For example a 660V fuse used on a 130V AC system would produce an arc voltage of 0.7 Un = 460V (130V = 0.2 x 660V)
FUSES IN PARALLEL
The I2t of the combination of fuses is the I2t of the single fuse multiplied by the square of the number of parallel fuses, ie by 4 for two fuses in parallel or by 9 for 3 fuses in parallel.
To ensure good current sharing between fuses, all connections should be as symmetrical as possible.
It is prudent to de-rate the current rating of parallel fuses by 5% for each parallel path.
The time – current value of the parallel fuse combination is the time – current curve of the single fuse multiplied by the number of parallel fuses, ie by 2 for two fuses in parallel or by 3 for three fuses in parallel.
Only identical types and values of fuses should be used in parallel.
RMS FUSE CURRENT
Fuses are selected for both their current rating as well as their I2t figure. The I2t figure is selected based upon the semiconductor being protected.
The normal operating current is the RMS current through the fuse. In various configurations of semiconductors and fuses, the RMS current can vary significantly. Some examples are shown in figure 1.
RMS FUSE CURRENT
Figure 1
TIME – CURRENT CURVE AND FUSE DATA
Figure 2
Typical Fuse Data:
Fuse type 50FE
Rating (Amps) 50
Watts dissipated @ In (W) 11
I2t pre arc (A2s) 103
I2t at 415V (A2s) 380
I2t at 660V (A2s) 600
WORKED EXAMPLE #1
75 Amp Solid State Relay operating on a 415V supply, providing 45A AC to a load. I2t of the Solid State Relay is 5000 A2s.
From Figure 1 this is an example of ANTI PARALLEL connection with one fuse protecting both semiconductor devices.
Irms = Iload therefore we select a fuse with a current rating of greater than 45A
To protect the semiconductor we select a fuse that has an I2t that is less than 75% of the protected devices I2t.
Fuse 50FE is selected as its current rating is greater than 45A and its I2t is less than 75% of 5000 A2s.
WORKED EXAMPLE #2
Soft Starter for 3 phase induction motors operating on a 415V supply. Start current is 60A for 20 seconds. Running current is 15A. The SCRs used have an I2t of 3000 A2s.
From Figure 1 this is an example of ANTI PARALLEL connection with one fuse protecting both semiconductor devices, Irms = Iload.
This application is of repetitive overload, so we use the time – current curve (see Figure 2) and select a fuse such that at the 20 second point the required current of 60A is 60% of the value in the fuse’s time – current curve. 60A / 60% = 100A. We would select fuse 50FE as it has a value of 110A at the 20 second point on it’s time – current curve.
To protect the SCR we select a fuse that has an I2t that is less than 75% of the protected devices I2t.
Fuse 50FE is selected as its current rating at 20 seconds is greater than 100A and its I2t is less than 75% of 3000 A2s.
WORKED EXAMPLE #3
Three Phase Bridge operating on a 415V supply, providing 55A DC to a load. The fuses are to be installed on the three AC lines into the Three Phase Bridge. The SCRs used have an I2t of 1000 A2s.
From Figure 1 this is an example of 3 PHASE BRIDGE connection.
Irms = 0.82 x Iload = 0.82 x 55A = 45A, therefore we select a fuse with a current rating of greater than 45A
To protect the SCR we select a fuse that has an I2t that is less than 75% of the protected devices I2t.
Fuse 50FE is selected as its current rating is greater than 45A and its I2t is less than 75% of 1000 A2s.
There may be situations, especially with devices that are water cooled, where a fuse cannot be found that will satisfy all the selection requirements. In this case it is recommended to use a larger semiconductor device.
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