Electronics in Modern Circuit Breakers 1

[Mbrooke]

Bit of curiosity. What is the highest molded case or PC breaker size a manufacturer sells without electronics? Just bi-metal and coil elements. I'm reading 225 amps but want to confirm.

 

[jraef]

Siemens will still sell you a Thermal-Mag breaker up to 2000A (the old ITE RD6 frame).


" We are all here on earth to help others; what on earth the others are here for I don't know." -- W. H. Auden

 

[Mbrooke]

Well, Siemens has my cash! And shhhh... its top secret



"Every person deserves love from other people"-

https://youtu.be/LyezyOrBEXk?t=218

 

[davidbeach]

Been many years since I worried about such things, I’m pretty sure that Square D offered a large frame thermal-magnetic breaker. Never saw the point. At that size I not only want to control the trip characteristics, I can’t imagine not needing to be able to do so.

 

[waross]

I understand that the inst trip settings are times the breaker frame rating, not times the replaceable trip unit rating.
That is, a 1000Amp breaker frame will have the same instantaneous trips no matter what the electronic trip is set at.
Eg;
A 1000 Amp breaker with a 1000 Amp rating plug will have the same instantaneous trips as the same frame with a 600 Amp rating plug.
Is this correct?


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Mbrooke]

@Waross: Good question.

One plus side: Thermal magnetic breakers are immune to EMP and failure of their electronics from surges. In applications where dependability takes precedence such breakers are IMO essential.

 

[xnuke]

I think one reason why large thermal-mag breakers aren't used very much anymore is that the instantaneous trip makes downstream coordination difficult if not impossible for large fault currents. Coordination isn't needed if used as a branch circuit breaker for a large load, but is desirable if used as a feeder breaker, and feeder applications are way more often encountered than large branch circuit load applications for a breaker with a high rating.

xnuke
"Live and act within the limit of your knowledge and keep expanding it to the limit of your life." Ayn Rand, Atlas Shrugged.
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[Mbrooke]

But couldn't you get the same magnetic curves with an adjustable spring as you would with electronics?

 

[LionelHutz]

Bill, any electronic breaker I'm familiar with has a trip level and time setting for the instantaneous trip and it seems to be based on the trip unit rating.

 

[dpc]

The range of the instantaneous pickup setting does change with the trip rating I believe. The advantage of the digital trip unit is mainly its accuracy and repeatability when compared to the magnetic trip.

A MCCB with digital trip isn't going to coordinate much that better than one with magnetic trip since both must have instantaneous. To truly coordinate with downstream MCCBs or fuses a low voltage power circuit breaker is a much better option.

I might also take some issue with the idea that thermal magnetic breakers are less susceptible to EM interference. Probably true 30 years ago, but not sure if that's still true. I can say with certainty that the old magnetic instantaneous trip units on IAC relays can misoperate due to energy from a 5-watt FM handie-talkie held close to the relay. I've not heard of misoperation of digital trip units due to radiated EM energy in a long time. But I don't get out much these days.

 

[waross]

Thanks for the update, Lionel and dpc.
It's been a long time since I was inside a breaker.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[che12345]

Dear Mr. Mbrooke

Q. ... What is the highest molded case or PC breaker size a manufacturer sells without electronics? Just bi-metal and coil elements. I'm reading 225 amps..."
A1. "Just bi-metal and coil elements " is termed "thermalmagnetic" by some manufacturer. The "thermal" is by (bi-metal) for [over-load], while the "magnetic" is by (electromagnet) for [short-circuit] protection. They are suitable for ac or dc networks. Numerous MCCB manufacturers with "thermalmagnetic" release up to 630A; in the IEC world. As for PC breaker = ACB (per IEC), "electronic" release prevail.
A2. For MCCB, the "thermal" is usually adjustable from say 0.6... 1.0In. While the " magnetic" may be fixed at 3,5,10In etc or adjustable 5-10, 6-14In etc., where In=rated current (of the breaker). e.g. breaker In=630A, the "thermal" may be adjustable from say 0.6 ....to 1.0In. The "magnetic" fixed at 5 or 10In or adjustable from 5...10In.
A3. For ACB:
a) electronic release may encounter problem to operate at very low say 1 to 5%In, e.g. for earth fault protection, as the voltage generated by the internal CT may not be sufficient to operate the electronic release. This can be circumvented by having external dc auxiliary supply to ensure that the voltage is sufficient to operate correctly.
b) there is no serious problem reported " due to radiated EM energy in a long time ". This is evident that electronic release prevail on the market.
Che Kuan Yau (Singapore)

 

[Mbrooke]

certainty that the old magnetic instantaneous trip units on IAC relays can misoperate

These have electronics, right? I ask because you say magnetic.

 

[davidbeach]

An IAC is a classic Electromechanical relay of 1950s technology, if not '40s or '30s. They might have said electronics at the time, if the term even existed, but it wouldn't have meant what anybody would understand it to mean today.

 

[waross]

Magnetic did not mean electronic.
It was a coil in series with the load current that developed enough magnetic force under over current conditions to mechanically trip the breaker.
In some designs it was arranged to increase the opening force of the contacts.
Some molded case breakers had a common trip bar. Force on the trip bar would cause the breaker to trip.
The magnetic instantaneous coils acted on the trip bar.
The bimetal strips of the thermal section acted on the trip bar.
and
In fuse protected breakers, the striker pins in the fuses acted on the trip bar.
The fuse could clear a single phase fault and the trip would clear the other phases to avoid single phasing.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Mbrooke]

I can't picture a walkie talky tripping that though.

 

[dpc]

I can't picture a walkie talky tripping that though

But it did. Many relay panels had warning signs regarding keying a radio too close to the relay.

No electronics in the old IAC or CO relays.

 

[waross]

Did that breaker have a shunt trip coil, dpc?


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[Mbrooke]

Errr... how does low level RF trip one of those?

 

[dpc]

Bill,

It was a protective relay, not a circuit breaker. The instantaneous trip element is a small induction cup or cylinder type device mounted very close to the front of the case. They are somewhat sensitive. You can also get them to trip by slamming the door the relay is mounted on or bumping the relay pretty firmly with your fist or hand. I think the GE IFC vintage improved on the resistance to vibration but I don't know about the RF issue. The shielding isn't great due to the glass front.