[brianafischer]

I am designing a control panel that includes a VFD and a SCR Power controller. The components/control panel has the following specs:

3 phase 480VAC
VFD: 3HP Saftronics CV104003 (5.2 Amps)
Power Controller: Omega SCR39Z (22.5 Amps)

I have 2 questions:

• Is there any reason, besides cost, why people use fuses instead of circuit breakers?
  
• The manual for the Saftronics drive states the Circuit Breaker should be 15A. Isn't this extremely over-rated, considering the drive consumes 5.2 Amps?

Thanks!

[TWControls]

brianafischer, on Sep 11 2006, 04:10 PM, said:

1. Is there any reason, besides cost, why people use fuses instead of circuit breakers?

For an OEM it is for the protection of the devices. This is less of a concern now than it used to be. If you are the end user then Arc Flash protection and lowering the incident energy can be achieve by installing fuses instead of breakers. It has to do with the reaction time. A fuse reacts in about .02 of a second. A breaker reacts in about .1 seconds. Those numbers are completely off of had but is a close representation

Quote

2. The manual for the Saftronics drive states the Circuit Breaker should be 15A. Isn't this extremely over-rated, considering the drive consumes 5.2 Amps?

It has to do with the inrush on power up when the capacitors are charging

[BobLfoot]

With the new emphasis on Arc Flash standards many companies are using fuses rather than breakers to reduce worker exposure to arc flash. A 15 Amp fuse has a significantly lower arc flash than a 15 amp breaker.

[Snerkel]

Also for SCR you should look at using Semiconductor Fuses for the best level of protection.

[TWControls]

Snerkel, on Sep 11 2006, 06:41 PM, said:

Also for SCR you should look at using Semiconductor Fuses for the best level of protection.

I hadn't thought about that. Could you elaborate on what type of fusing it should be?

[brianafischer]

TWControls, on Sep 11 2006, 06:45 PM, said:

Snerkel, on Sep 11 2006, 06:41 PM, said:

Also for SCR you should look at using Semiconductor Fuses for the best level of protection.

I hadn't thought about that. Could you elaborate on what type of fusing it should be?

This is what I know so far. Shamelessly ripped from the Omega SCR39Z manual

Quote

3.23 Fuses and Safety Warnings
Only I^2 fuses should be used for protecting the power controller's SCRs. These fuses are especially designed to protect the solid state devices under short-circuit conditions; other fuses may not act quickly enough. If it becomes necessary to replace a fuse, use only a Chase-Shawmut Form 101 or semiconductor fuse, or equivalent. IMPORTANT S A F t WARNINGS - READ BEFORE OPERATING CONTROLLER.
a Standard fuses or a circuit breaker should be used on all power lines for safety and to meet electrical code requirements. The supplied fuses are for protecting the SCRs only and are not acceptable as power line fuses. SCR power controllers do not satisfy electrical code disconnect requirements in the non-conducting or OFF state. Because they are semiconductor devices, the have a leakage current in the OFF state on the order of I OmA at rated line voltage. Therefore, the controller should be connected to a circuit breaker or disconnect switch. a SCRs can fail in a "shorted-closed" mode, resulting in full application of power. Use of a seperate, thermally protected safety contactor is strongly recommended.

The SCR39Z has a 40A internal fuse. My plan is to use a 30A circuit breaker. on the 3 phase 480VAC circuit with a 16kW heater and SCR39Z:

16kW * 1.25 = 20kVA
3 Phase x 20kVA x 460VAC = 25.13A
25.13A x 1.25 ~ 30A circuit breaker

This post has been edited by brianafischer: 11 September 2006 - 07:44 PM

[Snerkel]

Using a breaker as you suggest will be fine.

I assume the SCR is being used for a heater system? If it is then you will probably want to look at over temperature protection that controls a contactor in series with the SCR.

In normal operation the contactor remains on and the SCR controls the heating, if an over temperature is detected then the contactor is used to isolate the heating circuit.

Another idea for the over temperature protection is to use a circuit breaker with a shunt trip such as this http://new.kimcontro...3P030STDC?man=3
Putting 24v onto the shunt trip turns off the circuit breaker so an external circuit (such as over temperature) can be used to "trip" the breaker.

[brianafischer]

Snerkel, on Sep 12 2006, 03:04 AM, said:

Using a breaker as you suggest will be fine.

I assume the SCR is being used for a heater system? If it is then you will probably want to look at over temperature protection that controls a contactor in series with the SCR.

In normal operation the contactor remains on and the SCR controls the heating, if an over temperature is detected then the contactor is used to isolate the heating circuit.

Another idea for the over temperature protection is to use a circuit breaker with a shunt trip such as this http://new.kimcontro...3P030STDC?man=3
Putting 24v onto the shunt trip turns off the circuit breaker so an external circuit (such as over temperature) can be used to "trip" the breaker.

I just checked out the data sheet from here the shunt trip option seems like a good idea to save panel space.

[paulengr]

brianafischer, on Sep 11 2006, 03:10 PM, said:

• Is there any reason, besides cost, why people use fuses instead of circuit breakers?
  
• The manual for the Saftronics drive states the Circuit Breaker should be 15A. Isn't this extremely over-rated, considering the drive consumes 5.2 Amps

With a motor that draws say 10 amps, you can use a current limiting fuse at just a little over that. For a circuit breaker, you'd have to set it to the inrush, several TIMES the normal current draw of the motor. Effectively, circuit breakers will barely catch short circuits and even then only after your motor has been destroyed. Current limiting fuses will protect a motor from short circuit conditions with no damage.

With arc flash considerations, things are a bit more confusing. As has been previously mentioned, in many cases, fuses are a much better choice. However, there are some special cases I've seen where a circuit breaker works better than a fuse.

There is one more safety reason why fuses are preferred over circuit breakers. Technically, you get caught in a catch-22 situation with a circuit breaker. With a fuse, they only trip when there is actually a problem in the circuit. There is no such thing as a false trip with a fuse. With a circuit breaker, false trips are a fact of life. So when the circuit breaker trips, you don't know if there really was a fault or not.

This brings up an NEC rule. You are not supposed to reapply power to a circuit until you have positively identified and removed the fault from the circuit. With a fuse, you know a fault occurred. With a circuit breaker, you can't really ever determine whether it is safe to reenergize a circuit or not.

Worse yet, utilities actually set their circuit breakers to automatically reset at least once automatically before remaining open. They are actually covered under the NESC though instead of the NEC.