Fuse for solid state relay protection - semiconductor I2T or standard

[Steve]

For protection of hockey puck style solid state relays (say 10 A at
125 V), it is suggested by vendors to use semiconductor I2T type fuses
because they blow in 2 msec which is much faster than standard fast
blow fuses. But since the semiconductor fuse can cost more than the
solid state relay, I am wondering how important it really is to use
them, specifically for protection against shorted loads where the
relay would be subject to very high currents.
http://www.power-io.com/products/fuses.htm

 

[Phil Allison]

"Steve"

For protection of hockey puck style solid state relays (say 10 A at
125 V), it is suggested by vendors to use semiconductor I2T type fuses
because they blow in 2 msec which is much faster than standard fast
blow fuses. But since the semiconductor fuse can cost more than the
solid state relay, I am wondering how important it really is to use
them, specifically for protection against shorted loads where the
relay would be subject to very high currents.

** I suggest you need to *eliminate * short circuit load events - only
sheer carelessness allows them to happen.

A 10amp SSR will typically withstand 100 to 150 amps for one cycle - enough
time for a fast HRC fuse or magnetic breaker.

Possibly add series resistor into the circuit or a non-saturating inductor
of 1.5 mH to get the short circuit current down.


...... Phil

 

[JosephKK]

For protection of hockey puck style solid state relays (say 10 A at
125 V), it is suggested by vendors to use semiconductor I2T type fuses
because they blow in 2 msec which is much faster than standard fast
blow fuses. But since the semiconductor fuse can cost more than the
solid state relay, I am wondering how important it really is to use
them, specifically for protection against shorted loads where the
relay would be subject to very high currents.
http://www.power-io.com/products/fuses.htm

Do consider the consequences of various failures. Blown fuse, down
for an hour. Blown semis, down for a day or two. Blown motor, down
for weeks. Or maybe completely different.
But only you and your customer really know the application and the
consequences, that is part of the engineering.

 

[lurch]

Do consider the consequences of various failures. Blown fuse, down
for an hour. Blown semis, down for a day or two. Blown motor, down
for weeks. Or maybe completely different.
But only you and your customer really know the application and the
consequences, that is part of the engineering.

Monitor the desired switched line in question. Monitor the voltage on
that shunt. If the max value is reached, "OPEN" the low voltage DC
excitation line that activates the relay, NOT the line experiencing the
high current.

In other words, turn off the relay, instead of attempting to open the
line the relay switches with yet another set of contacts of some sort (a
breaker).

The relay is already the switch/circuit/line breaker.

 

[Phil Allison]

"lurch" <[email protected]

Monitor the desired switched line in question. Monitor the voltage on
that shunt. If the max value is reached, "OPEN" the low voltage DC
excitation line that activates the relay, NOT the line experiencing the
high current.

In other words, turn off the relay, instead of attempting to open the
line the relay switches with yet another set of contacts of some sort (a
breaker).

** Nice idea - but it cannot work with short circuits. Problem being, that
one cannot instantly turn off a triac or normal SSRs by removing drive.

It will take unit the end of the half cycle ( ie the current must fall to
near zero) for the triac to commutate off - so the full fault current will
have to be tolerated for that long at least. Good bye 10 amp triac.

For your idea to work, impedance must be added in series with the load to
limit the fault current to a safe value. Plus the circuit must latch off and
be manually reset when the fault is cleared.

Much more practical to just use an up-rated SSR, add a bit of series
impedance and fit a fast acting HRC fuse.

Works for professional lighting dimmers.


..... Phil