Static circuit breakers

Triacs are completely unsuitable for this role as they must wait for a zero crossing to stop conduction. In this time the prospective short cicuit current can build up considerably and damage the prtected device. They also don't work at all with DC currents.

FETS or IGBJTS can instantly halt an over current though they must be considerably over sized as the I^2T ratings are more restricted.

It was my understanding that silicon carbide based FETS would make electronic short circuit limitation quite cost effective becuase the higher junction temperature would allow superior over current capabilities.

As far as I can see moder IGBJTR modules have in built over current protection that activtaes when the Collector-Emitter voltage excedes the standard drop by more than switching time so some sort of over current cicuit is unlikely to be needed.

Sounds good. So.. does anyone make these into little DIN-rail mounted circuit breaker module thingys with open/close/trip terminals that I can hook cables up to?

Thanks, Cameron:-)

I'll de-lurk again (twice in one year!).

OK, sounds like you want a remote operation and indication for a standard minature circuit breaker. I'm sure I've plugged these products before in here, but they bear repeating. Don't forget also that MCBs typically work with both AC and DC provided you deploy them correctly and follow the calculations of the manufacturer.

GE make a motorised operator for standard DIN rail mount MCBs. They also make aux contact blocks with changeover contacts to allow you to remotely interogate the breaker status.

I could be wrong with these numbers, but from a well known UK supplier:

260000 Motor Operator 240V 268907 EP Aux Contact 'SH/HH'

An example breaker...

268391 MCB 6A 3P D 6KA 3M

Currently in use in a "panel" that is 30 metres under-water in an installation of mine. Remote reset is a necessity otherwise you need SCUBA gear to ge to the "panel" door - actually the pressure vessel lid!

Back to lurking...

Dave Slee The Engineering Business Ltd.

Thanks, Dave - I'll definitely have a look. FYI, Terasaki's DINT41, Moeller's RE-PKZ2 and M-G's Tn operators are similar sort of things, but I am glad to have something else to add to the list for "Plan B".

Sorry to "de-lurk" you again, but have you ever had any problems with these motorised operators? They are extremely expensive (a $200 operator for a $20 breaker) and you're the first person I know that is using them.

With all this new-fangled Drive technology I was hoping someone might have applied it to motorised circuit-breaker use. It would be cheaper, smaller and (quite possibly) more reliable - motors do burn out, after all...

Cameron:-)

I haven't seen any trouble yet, although that statement is almost certain to cause them to fail in the early hours of the morning tomorrow! The breaker I mentioned in the previous post has been in service over a year now in an application where it has been largely on stand-by.

I don't want to use them either for all the reasons you give, but sometimes I'm constrained in design to solutions I don't really like. I'm sure it's the same for everyone.

Me too. However I still like the idea of a physical gap in a device and a padlocking position for my lock out kit when I am working down-stream of a protective device. "Reliable" is a very loaded word - I want my breaker to trip and stay tripped every time a fault condition persists. I'm less worried about it energising the cicuit every time I command it. In the remote operation scenario, failure to clear the fault might lead to loss of life, whereas failure to energise the load might mean some expensive travel and some plant down-time.

In that context it is more understandable that products employing semiconductor isolation are few and far between. I guess in these litigous times, the risk that a semi device might shoot through and nail a technician is managed by putting those old fashioned, comfortable air gaps in the products.

Just for fun, I know that Rockwell are selling a "solid state contactor" in place of star-delta-line contactors for motor starting applications (there are plenty of others, just the data sheets are to hand for A-B). I'll just take a look and see if they recommend that it is backed off by a mechanical means of isolation ...

Here's the relevant safety warning...

"When not using an isolation contactor, hazardous voltages are present at the load terminals of the controller when the controller is turned off. Warning labels must be attached to the motor terminal box, the controller enclosure, and the control station. Additional equipment and circuitry must be included to provide automatic isolation."

I've played around with all sorts of schemes for detecting over-load and recovering from it gracefully over the years. My current favourite approach to those "tricky" single phase non-motor problems is a mechanical isolator and gross over current protection from something like an MCB or MCCB chosen so it doesn't trip unless it's a complete disaster (spanner on busbars, fire in the panel etc.) and then sense current for close protection using a Hall effect CT with a 4-20mA RMS value output and a trip amplifier driving a contactor. The price of the Hall effect CTs continues to tumble. There are also a number of very good all-electronic replacements for the traditional contactor mounted heater and bi-metal strip overload out there if motors and heaters are your thing.

I suppose the key is to separate the protective function from the control function and address each one with the appropriate tool - air gap for the protection, semiconductors for the control. Any "protection" duties that the control layer can't handle escalate up to a device that *can* deal with the problems. It requires another layer of co-ordination though.

Dave Slee The Engineering Business Ltd.

I have to agree here. That is why I hate triac outputs from PLCs. I specify dry contact, reed relay outputs for all my DCS and PLC applications. They work on AC, DC and milliamp circuits. They don't leak. The first time I encountered triac leakage is when I had to put the output of one PLC into the input of another. The circuit would NOT turn off until I added a 5 W, 5K Ohm resistor in parallel with the input. That seemed to hold the outputs down when they were supposed to be off.

I always thought there was something wrong with a system whose outputs were incompatible with its own inputs.

Walter.

There may be an alternative. Syrelec make a current monitoring relay. (I think they are now part of Crouzet now but other companies also make these)

The DIRT2 and DIRT2D relay can monitor anyting from 5mA AC or DC to

10A AC or DC on the basis of a dial and a range selection. A second dial adjusts the time delay for on delay or of delay.

Anything above 10Amps will require a current transformer.

It can be wired in a non-latching and a latching mode (you need the latching mode) and in the latching mode can be re-set remotely.

The contacts are only signaling contacts rated to 10A AC1 but in combination with an oversized overload protected contactor could provide an effective over current protection.

10 Amps, hey? I'm onto it right now! Thanks heaps!!

Cameron:-)

This is the data page for them: