Fuse on Primary Side of power supply

Good afternoon all, I'm looking at the PSB24-120S-3 power supply from automation direct. The recommended circuit breaker size on primary size is 16A per phase. However, the in-rush current is stated as "<30A @ 400VAC & 500VAC @ 25°C (With 3Ph AC source capability up to 3KVA) <60A @ 400VAC & 500VAC @ 25°C (With 3Ph AC source capability up to 18KVA)".

Since this power supply has the potential to have max-inrush current at 30A, shouldn't I use 30A breaker or more if I'm pulling max load on the secondary side? In this case, max Amperage will be 5A on the secondary side, so if I'm using all of that, then I would need higher breaker on primary side, correct? Looking to improve my understanding on how to correctly size the power supply on my application.

Thanks,

A normal 16 amp circuit breaker does NOT trip at 17 amps as a lot of people think.

Most are designed to trip at double the rated current instantly. So a 16 amp breaker that trips at 32 amps would allow a 30 amp inrush.

Oh wow, that's really interesting, but also very confusing. So it can withstand 17A for a couple of seconds before tripping? Or if it goes above 32A, then it will instantly trip? Does that only apply for circuit breakers, or fuses as well?

Thanks,

There are different types of circuit breakers suitable for different devices. Type B for lighting, Type C for fans, ballasts, and air conditioners, and Type D for large motors and transformers.

"The MCB with class B trip characteristics trips instantaneously when the current flowing through it reaches between 3 to 5 times rated current. These MCBs are suitable for cable protection."

Circuit breakers and fuses both have trip curves that represent how they will react to amperage values over time.

https://www.c3controls.com/white-paper/understanding-trip-curves/

https://www.eaton.com/Eaton/Product...cumentation/TimeCurrentCurves/Fuses/index.htm

Thanks for all the info everyone! I understand it better now.

Bit_Bucket_07 said:

Circuit breakers and fuses both have trip curves that represent how they will react to amperage values over time.

https://www.c3controls.com/white-paper/understanding-trip-curves/

https://www.eaton.com/Eaton/Product...cumentation/TimeCurrentCurves/Fuses/index.htm

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Also interesting to know is that curves overlap and not how one would assume. A B16 breaker trips before a C10 (or something like that, I can't find the curves in google right now).

All explained in power supply installation instruction sheet:-


5.1. Input connection (Fig. 1, Fig. 5)Use L1, L2, L3 with GND connections of input terminal connector (See Fig. 5) to establish 3 x 400-500VACconnection. Fig. 5 shows the connection to the various network types. The power supply is suitable for use with 3-phase star network power grids only.In the event of a phase failure, unrestricted operation is possible with nominal capacity. The unit is protected with internal fuse (not replaceable) at L pin and it has been tested and approved on 20A (UL) and 16A (IEC) branch circuits without additional protection device. An external protection device is only required if the supplying branch has an ampacity greater than above. Thus, if an external protective device is necessary, or, utilized, a minimum value of 13A B- or 6A C- characteristic breaker should be used.

What cardosocea describes is called "discrimination", although give it another few years and that won't be a politically correct name any more.

Discrimination is when breakers are selected such that the downstream breaker will always trip before the upstream one, i.e. the little 16A one in your panel will trip before the 100A one on the feeder supplying it. For standard C curve breakers, 2 sizes difference between breakers generally gives you discrimination. However, when you start mixing different trip curves and breakers with adjustable curves then you really need to use the software from the breaker manufacturer to figure this out.

If all breakers in a system will trip in the correct order then it is said the system has full discrimination. If under some conditions, i.e. short circuit, the curves overlap then the system is said to only have partial discrimination.

In your application, the breaker is sized primarily to protect the wire AND to meet the PS manufacturer installation conditions. That inrush current is only present for a few milliseconds and isn't of consideration for sizing the wire and breaker for steady state, continuous operation. However, it might cause nuisance trips if the breaker is too small.

For high inrush current devices like motors, transformers and some power supplies, I often use a D curve breaker that has a higher instantaneous trip threshold, but still the same long time trip rating of a C curve breaker.

My theory is don't fuse anything. You could damage a fuse. What a terror that would be. I say: " Protect that fuse!" at all costs.

lee1968nh said:

My theory is don't fuse anything. You could damage a fuse. What a terror that would be. I say: " Protect that fuse!" at all costs.

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You must have shipped a lot of panels to the Detroit area.

I see your theory at work almost every week.

You have to remember that the circuit component and wire damage is a result of heat, and heat buildup isn't instantaneous. Devices like fuses and circuit breakers have time delays. The higher the current the faster the heat buildup and the sooner the protection trips.

I don't like Detroit very much. Livonia, on the other had is where I used to live. If it's Livonia, use a fuse.