Short circuit protection
Within the framework of the European Low Voltage Directive 2014/35/EU (Annex I, article 3, point c), it is necessary to protect your installation against the dangers that can be caused by external influences on the electrical equipment.
“Technical measures are provided for in accordance with point 1 (particularly point c). The electrical equipment is designed and manufactured in such a way that protection against the dangers mentioned in points 2 and 3 is guaranteed, subject to use for its intended purpose and adequate maintenance.), so that the electrical equipment does not not endanger people, pets or objects under the overload conditions foreseen. »
The perfect protection of a solid state relay against short circuits or overcurrents requires good technical knowledge. In this article we give you the key elements to know for good protection.
There are 2 types of current protection:
BY FUSE: to protect the static relays against load short-circuits, fuses must be used, in particular ultra-fast for small ratings. The I²t value of the fuse must be less than half the I²t value of the relay.
BY CIRCUIT BREAKER: this mode of protection can be adapted to static relays with an I²t value > 5000A²s.
First, let's start with a reminder of what the IEC standard says in terms of coordination levels:
1- What does the IEC standard say in terms of coordination with short-circuit protection devices?
Short-circuit coordination is the combination of a relay and protection against short-circuits:
To obtain desired protection, safety (type 1) or relay (type 2)
For a potential current (if there is no protection) of maximum short-circuit also called prospective short-circuit current. It depends on the point of the electrical installation
Only coordinations tested by celduc relais are guaranteed.
IEC publication 60947-4-x distinguishes between two different types of protection, (called “coordination”), which are designated types “1” and “2”.
Type 1 coordination requires that under short circuit conditions the device does not cause danger to persons or the installation and may not be in working order for other services without repair or replacement of parts .
This means that in the case of static relays with loads where the risk of short-circuiting the load is low, this type of type 1 coordination is well suited. In this case, the installer simply has to choose his protection to protect the installation and people. On the other hand, in the event of a short-circuit, the protection device, as well as the relay, must be changed.
Type 2 coordination requires that, under short-circuit conditions, the device does not cause danger to people and the installation and must be suitable for subsequent use. For hybrid dimmers and contactors, the risk of contact welding is accepted, in which case the manufacturer must indicate the measures to be taken for the maintenance of the equipment.
This means that in the case of solid state relays with loads where the risk of short-circuiting the load is higher, this type of coordination type 2 must be chosen. In this case, the installer must adapt the relay and the protection so that the relay remains operational after a short circuit. The following pages of this document will help you to correctly determine the protection according to the type of relay, but also to the prospective short-circuit current of the installation.
2-Protection against short circuits recommended by celduc
On the celduc relay data sheets we give information on the rated conditional short-circuit current and the type of coordination of the relay as well as the rated current and the characteristics of the associated Short-Circuit Protection Device.
The prospective short-circuit current is given in the technical documentation of the fuse.
Now here is more information on the general method of determining the correct fuse for the solid state relays you are using in your application.
Protection by ultra-fast fuses
Standard fuses cannot react quickly enough to prevent the fault current from exceeding the maximum levels the thyristors can withstand. This is the reason why we recommend the use of ultra-fast fuses.
In our datasheets and selection guide, we specify the I²t value (amps square seconds) which is the maximum current versus time the thyristors can withstand.
For coordination level type 2, the “I²t” rating of the selected fuse must be less a nominal I²t value of the selected solid-state relay. To have good type 2 coordination, celduc relais recommends the following rule:
I²t fuse < ½ I²t typical of the solid state relay
The fuse must be installed upstream of the SSR. Indeed, if for an undetermined reason the relay must break the insulation to the ground (overheating, breakage of the casing, leakage with the dissipator, etc.), the fuse must protect the entire circuit.
Miniature circuit breaker protection
Do you often ask us if you can use an MCB instead of a fuse to protect both the circuit and the SSR? (Type “2” coordination). The answer is “YES” but you must consider the I²T parameters of the SSR, the total short circuit current available in the system, and of course also select a circuit breaker with a sufficiently lower I²t than the SSR.
3- Need help defining the right protection for your installation?
The celduc relais R&D department is at your disposal to help you choose the right combination of solid state relays and protection against short circuits.
You can choose your protection coordination yourself, but you then take responsibility for it because only the coordinations tested by celduc relais are guaranteed.
4- What to do after a short circuit?
During a short circuit on an equipment, some damage may appear on an installation.
First, you have to check the origin of the short circuit and make the necessary repairs.
Then you have to inspect the installation to check if other damage has not been done.
Attention, even with correct protection, the number of short-circuits for a semiconductor device is limited.
Repeated short circuits can decrease the life expectancy of the semiconductor.
Similarly, the number of short circuits for a circuit breaker is limited.
We recommend :
1-Not to subject a static relay to more than 10 short circuits during its entire life
2-Before putting the protective device back in working order, check the operation of the static relay.
3-If in doubt, it is best to replace the circuit breaker on the one hand and the relay or static contactor on the other.
5-Conclusion
Having the right overcurrent protection is essential. Thyristors, which are power elements of solid-state relays, have very short thermal time constants. Therefore, extreme current levels and overvoltages caused by load or line faults, even if only applied for extremely short periods of time, can cause permanent failure of thyristors.
celduc, as a manufacturer of solid state relays, as well as fuse or MCB suppliers, can both give you advice. Do not hesitate to ask us. It is important to note that the coordination level and short circuit tests must be carried out by the panel builder.
Short circuit protection
Within the framework of the European Low Voltage Directive 2014/35/EU (Annex I, article 3, point c), it is necessary to protect your installation against the dangers that can be caused by external influences on the electrical equipment.
“Technical measures are provided for in accordance with point 1 (particularly point c). The electrical equipment is designed and manufactured in such a way that protection against the dangers mentioned in points 2 and 3 is guaranteed, subject to use for its intended purpose and adequate maintenance.), so that the electrical equipment does not not endanger people, pets or objects under the overload conditions foreseen. »
The perfect protection of a solid state relay against short circuits or overcurrents requires good technical knowledge. In this article we give you the key elements to know for good protection.
There are 2 types of current protection:
BY FUSE: to protect the static relays against load short-circuits, fuses must be used, in particular ultra-fast for small ratings. The I²t value of the fuse must be less than half the I²t value of the relay.
BY CIRCUIT BREAKER: this mode of protection can be adapted to static relays with an I²t value > 5000A²s.
First, let's start with a reminder of what the IEC standard says in terms of coordination levels:
1- What does the IEC standard say in terms of coordination with short-circuit protection devices?
Short-circuit coordination is the combination of a relay and protection against short-circuits:
To obtain desired protection, safety (type 1) or relay (type 2)
For a potential current (if there is no protection) of maximum short-circuit also called prospective short-circuit current. It depends on the point of the electrical installation
Only coordinations tested by celduc relais are guaranteed.
IEC publication 60947-4-x distinguishes between two different types of protection, (called “coordination”), which are designated types “1” and “2”.
Type 1 coordination requires that under short circuit conditions the device does not cause danger to persons or the installation and may not be in working order for other services without repair or replacement of parts .
This means that in the case of static relays with loads where the risk of short-circuiting the load is low, this type of type 1 coordination is well suited. In this case, the installer simply has to choose his protection to protect the installation and people. On the other hand, in the event of a short-circuit, the protection device, as well as the relay, must be changed.
Type 2 coordination requires that, under short-circuit conditions, the device does not cause danger to people and the installation and must be suitable for subsequent use. For hybrid dimmers and contactors, the risk of contact welding is accepted, in which case the manufacturer must indicate the measures to be taken for the maintenance of the equipment.
This means that in the case of solid state relays with loads where the risk of short-circuiting the load is higher, this type of coordination type 2 must be chosen. In this case, the installer must adapt the relay and the protection so that the relay remains operational after a short circuit. The following pages of this document will help you to correctly determine the protection according to the type of relay, but also to the prospective short-circuit current of the installation.
2-Protection against short circuits recommended by celduc
On the celduc relay data sheets we give information on the rated conditional short-circuit current and the type of coordination of the relay as well as the rated current and the characteristics of the associated Short-Circuit Protection Device.
The prospective short-circuit current is given in the technical documentation of the fuse.
Now here is more information on the general method of determining the correct fuse for the solid state relays you are using in your application.
Protection by ultra-fast fuses
Standard fuses cannot react quickly enough to prevent the fault current from exceeding the maximum levels the thyristors can withstand. This is the reason why we recommend the use of ultra-fast fuses.
In our datasheets and selection guide, we specify the I²t value (amps square seconds) which is the maximum current versus time the thyristors can withstand.
For coordination level type 2, the “I²t” rating of the selected fuse must be less a nominal I²t value of the selected solid-state relay. To have good type 2 coordination, celduc relais recommends the following rule:
I²t fuse < ½ I²t typical of the solid state relay
The fuse must be installed upstream of the SSR. Indeed, if for an undetermined reason the relay must break the insulation to the ground (overheating, breakage of the casing, leakage with the dissipator, etc.), the fuse must protect the entire circuit.
Miniature circuit breaker protection
Do you often ask us if you can use an MCB instead of a fuse to protect both the circuit and the SSR? (Type “2” coordination). The answer is “YES” but you must consider the I²T parameters of the SSR, the total short circuit current available in the system, and of course also select a circuit breaker with a sufficiently lower I²t than the SSR.
3- Need help defining the right protection for your installation?
The celduc relais R&D department is at your disposal to help you choose the right combination of solid state relays and protection against short circuits.
You can choose your protection coordination yourself, but you then take responsibility for it because only the coordinations tested by celduc relais are guaranteed.
4- What to do after a short circuit?
During a short circuit on an equipment, some damage may appear on an installation.
First, you have to check the origin of the short circuit and make the necessary repairs.
Then you have to inspect the installation to check if other damage has not been done.
Attention, even with correct protection, the number of short-circuits for a semiconductor device is limited.
Repeated short circuits can decrease the life expectancy of the semiconductor.
Similarly, the number of short circuits for a circuit breaker is limited.
We recommend :
1-Not to subject a static relay to more than 10 short circuits during its entire life
2-Before putting the protective device back in working order, check the operation of the static relay.
3-If in doubt, it is best to replace the circuit breaker on the one hand and the relay or static contactor on the other.
5-Conclusion
Having the right overcurrent protection is essential. Thyristors, which are power elements of solid-state relays, have very short thermal time constants. Therefore, extreme current levels and overvoltages caused by load or line faults, even if only applied for extremely short periods of time, can cause permanent failure of thyristors.
celduc, as a manufacturer of solid state relays, as well as fuse or MCB suppliers, can both give you advice. Do not hesitate to ask us. It is important to note that the coordination level and short circuit tests must be carried out by the panel builder.
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