sizing of circuit breakers

[fuggeos]

We have an application where a universal input power supply with 48VDC output, 275 watts maximum input powering some instruments is to be supplied with 208VAC input via a long cable ( about 6500 ft' long ) using 2- #12AWG wires. The power supply will be mounted inside a subsea housing located in the seabed and the 208VAC power will come from the surface.

We estimate that the voltage at the input of the power supply will be about 170VAC, after taking into account the voltage drop in the line. With 275watts input drawn at 170VAC, the current is about 1.6 Amps, maximum.

I am planning to use a 5A instantaneous trip breaker topside for equipment protection. Is this the correct type of breaker to use or should we use instead a thermal breaker?

are there any other issues that we should be worried about aside from inrush current that we are trying to determine when we get hold of the instrument?

any help would be greatly appreciated.

 

[advidana]

I would use a Thermal magnetic breaker if available at 5 amps. The magentic part of the breaker is the instantaneous tip and the thermal is for the over current. This breaker is kind of a do all breaker and is the standard for most circuit breakers.

The lower voltage to the power supply may not work well. usually the power supplies are rated 208 plus or minus 10 percent or less. Check you power supply specification for input voltage.

yes there is a lot of issues if that device shorts out to the case underwater. What are you doing to protect the person that may touch that device underwater, etc.

 

[dpc]

That seems like an awfully long feeder at 208V single-phase. Especially submerged. You have to overcome 13,000 feet of cable impedance. I'm not sure anything will be coming out at the other end! You need to think about the capacitance of the cable as well as the resistance and inductance. There will be charging current due to the shunt capacitance of the cable.

Your voltage will vary significantly as the load changes, so make sure your power supply can handle a wide range of input voltage.

I agree that a thermal-mag breaker is probably a safer choice.

 

[fuggeos]

thank you dpc and advidana for the comments.

The power supply that we will be using is the universal input type, i.e., it will work from 90-260VAC input. we estimate the input power to the power supplies at 275 watts. Using #12 AWG, the calculated input voltage down there ( after the voltage drop) is about 170 VAC.

With regards to the capacitance and inductance of the cable, is there any way that I can calculate these charging currents as you said (dpc) , if I can get the capacitance /inductance data for the cable? Any suitable model to use? . Are there some rule of thumbs that you know about? This is my most serious concern really. I would like to make sure that this thing will work properly when deployed.

about the safety issue, the electronics housing will be at a water depth of 6000 ft when deployed some where in the gulf of mexico so accidental touching won't be an issue. On surface, while testing the unit, the housing will be connected to a very grounding wire as a standard safety precaution in case of accidental shorting of AC line to the housing.

Would be nice if I can do a mock up test, with inductances, capacitances and resistances to model the cable and determine beforehand if there will be a problem. Right now, I can only simulate the cable using the approximate line resistance of the cable.

Would you have any ideas on doing this?

 

[dpc]

If you know the exact cable you will be using, the cable manufacturer should be able to give you parameters for the that particular cable. The capacitive reactance may be give as ohm-feet or Megohms-1000 Feet. If so you need to **divide** by the length to get the shunt capactive reactance. The capacitance is dependent on the cable geometry, insulation type, thickness, etc.

The Standard Handbook for EE should have some general data on conductor/cable parameters that might be of use. I assume the cable is adequately designed to support its own weight and expected motion.

 

[rbulsara]

Consider using a fuse rather than a circuit breaker. As CB in low current range may not be easily available or not as accurate and reliable as fuses.

This of course assumes all other issues mentioned by precedeing responses are taken care of.

 

[peebee]

Why not use a 15 or 20 amp breaker? Why try to throttle it down so far? 15 is the smallest standard breaker rating so far as I know.

 

[busbar]

Some random thoughts… The circuit length may be so great that a short at the ‘receiving’ end will not circulate enough current to operate the overcurrent device. The overcurrent device would, however, react to faults closer to the ‘sending’ end of the cable.

 

[fuggeos]

on dpc's comments about the cable-

the cable will be double armored, so no problem in supporting its own weight. I can get the capacitance/ft of the cable but how do i model that accurately? is it a pi configuration ( 2 Cs and 1 R) or a T, or something else? or can I just lump it in one R and a C ( shunt, across the supply side or the instrument side?)