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Join Date: Oct 2007
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Inrush Current and StartUp Current

10/24/2007 5:03 AM

I got the specifications from Vendor for two different type of Server Equipment. One having the wording "Inruch Current (max.) < 85A Amp" while another "Startup Current (max.) < 100Amp". Are they referred to the same thing?

How come a circut breaker (say 20Amp) or MCB (say 60Amp) would not trip for such high current during the equipment power up concurrently?

2. Please advise?

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Power-User

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#1

Re: Inrush Current and StartUp Current

10/24/2007 5:27 AM

Yes.

It's to do with energy, yes the current is high but for a relatively short time so the amount of energy involved is not enough to trip the breaker (I^2*t). The back emf will build in the motor and limit the flow of current to the amount needed to drive the load. An overload causing a current increase will cause the overload device to trip if sustained. If the motor starts slowly due to a large load this may cause nuisance tripping of the breaker.

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Guru
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#2

Re: Inrush Current and Start-Up Current

10/24/2007 9:57 AM

No they are not the same. Inrush current is for normal operating condition.

Startup current is usually much higher but is for a specific time period starting at the initial application of power.

Circuit breakers (as well as fuses) are also rated by the time the high current must be maintained before it will trip. The rating is typically about 1 second for household use but can be designed for any time delay and should be matched to the application.

Some power supplies and circuit breakers will have a startup sequence for the initial application of power. An electrical motor when first energized will draw allot of current until it is up to speed. They are designed to ignore the initial high current condition for a fixed period of time. After the start up sequence is completed, the operating protection which is usually much lower then the start up current will take over.

Most electronics also will have built in protection during start up for the protection of sensitive circuits, however, during a normal operating environment, they are susceptible to surges.

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Power-User

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#3
In reply to #2

Re: Inrush Current and Start-Up Current

10/24/2007 10:49 AM

I still believe they are the same, would you clarify the distinction you make please it's not clear to me.

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#4
In reply to #3

Re: Inrush Current and Start-Up Current

10/24/2007 11:25 AM
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Associate

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#6
In reply to #2

Re: Inrush Current and Start-Up Current

10/24/2007 11:52 PM

Whoever said the two are different does not his basics. Inrush, startup and transient current are all same. This current is multifolds compared to the normal operating current and most of the power feeders like UPS, Inverters, Generators etc are designed to take such a shock. For ex. my 19" monitor draws a transient current of 27A whereas its norma operating current is less than 1A. In rush current or whatever you call it is because of the large value capacitors found in SMPS and the current drawn drawn by inductive loads before the reverse EMF limits the current. R. GANAPATHI RAO, BANGALORE, INDIA.

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#8
In reply to #6

Re: Inrush Current and Start-Up Current

10/25/2007 9:41 AM

"Inrush current and start-up current are the same thing". Usually, yes. But some very large systems stagger the starting of their fans during start-up checks, whereas several may all come on more-or-less simultaneously if different parts of the system happen to heat at the same time. In that case, start-up current can be smaller than the worst-case inrush. That is just an example, of course. Start-up current can impact the charges made for peak load factors; inrush would define circuit-breaker and UPS requirements, for example.

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Guru

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#5

Re: Inrush Current and StartUp Current

10/24/2007 7:41 PM

Techno basically answered your question. An electric motor at startup will more quickly blow a circuit than a light, but the wattage demands of the machine at the end of the line has its needs, and if its needs are greater than the circuit is able to supply, the circuit breaker trips.

Rule of thumb is that 1000 Watts is 10 amps of push, or supply, and if you plug something in that wants 20 amps, like a 2000 Watt light, you can get away with it in a 15 amp circuit for a fair amount of time, though the plug may melt and need to be replaced.

Electric Motors are more likely to trip a circuit at start-up than lights, but you will never go wrong if you know the demands of the machines or instruments you are putting online, and never starve them.

Actually a 2000 watt light wants 17.5 amps which is why the circuit with its slop that is factored from 117 to 123 volts generally takes a bit before it gets too hot to handle.

At least that is what I know from doing it.

I had to give a 10,000 watt light 100 amps was all I had to know to run a set of lights without starting a fire is what I know.

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Power-User

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#7

Re: Inrush Current and StartUp Current

10/25/2007 6:56 AM

I have looked at the links and can find no distinction within the documents, start up describes the time when inrush currents happen inrush currents happen at start up. The two terms describe the same thing. Transients however may happen at any time, a voltage spike on the supply etc. but would normally be for a shorter time, less than a cycle, than the start up time. The original questioner asked two questions, are they the same, yes and why doesn't the device trip when the current exceeds the indicated value, because it's designed (or in the case of techno's links, programmed) not to.

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#9
In reply to #7

Re: Inrush Current and StartUp Current

10/25/2007 10:43 AM

I'm with you HUX, the same happens when a motor operated valve is torque seated. When the wedge or globe hits the sit, the gears stop braking for a period of some tenths of a second, and the current rise up (normally less than inrush value) but the breaker doesn't trip. But if for example, you try to start and stop several times a motor in a short period of time, the breaker would trip. As someone have said is a question of energy.

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Guru

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#10

Re: Inrush Current and StartUp Current

10/25/2007 4:29 PM

inrush / startup current are different , transient current different from start up , inrush and transient currents are seen when powered to appliances till they achieve there peak performances , start up current is basic need or minimum amount of current to supply rated power , some books do define all three same but applications define them differently

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Power-User

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#11

Re: Inrush Current and StartUp Current

10/26/2007 4:44 PM

I really enjoy threads like this one. It brings to light so many differences that are sometimes hard to understand. For the most part no one is wrong...for their application and understanding. Sometimes the difference is simply wording or industry. Books vs. reality. Electronics vs. electrics.

The thread started with a question about server specs. Electronics. Ganapa used an electronics example that is (in my humble opinion) absolutely correct. In the electronics world in-rush...startup..whatever you want to term it is pretty well the same. You push the power switch on your monitor...you get in-rush current based on circuit design impedances..for a few charging cycles...then it settles down to operating currents...

Electrics ... now there's a different animal ... maybe just different definitions based on location, equipment, training, etc. In electrics in-rush and starting current are very different (again..in my humble opinion).

I might have to differ a bit with what Techno said although I think you mean exactly the same as I. Typically...in electrics ..motors, transformers , etc. in-rush is defined as the initial magnetizing current and is calculated 'typically' for 6x motor FLA. Transformers are a bit more tricky since inrush current can be a lot higher depending on the last magnetizing phase angle. Regardless..inrush is still the initial current based on circuit impedance at energization...same as in electronics.

But, if you were to use a scope on a motor you would see why we say inrush and startup are not the same. Stick a scope on your motor...pull in the contactor...wham ...very high current with very short duration. Afterall...a motor winding is just a short circuit until current causes a magnetic field to build. But the motor isn't even spinning yet, is it? Now comes startup current and time that is different from inrush and must be dealt with differently. Take a 1hp motor at 480 without anything attached to the shaft. Measure it with a scope. You have inrush ..then the motor spins up to speed and during that period decreasing curve current flows until at rated speed. Now put a load on. Inrush current/time stays exactly the same...but your overall starting current/time curve is totally different and is load based.

Anyway..just my humble opinion.

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Power-User

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#12
In reply to #11

Re: Inrush Current and StartUp Current

10/30/2007 4:43 AM

Thank you for clearing that up. That makes perfect sense, is simple and lucid and I am wiser for it. I can see the mistake in my mental model, now I have a better one.

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Anonymous Poster (1); ganapa (1); HUX (4); Kwetz (1); Switchman (1); techno (2); Transcendian (1); vikas (1)

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