Power Supply Protection

[Tim Ganz]

I have the power supply in the link below. If we want a breaker to isolate and protect the input side how would I size that correctly being 120 volts on the input and 1.25 amp 24 dc output?

https://cdn.automationdirect.com/static/specs/cutsheet/2687-2142_cutsheet.pdf

 

[parky]

2A type C would probably be the smallest you could use, as the PSU has protection it will depend on the wire size used to feed it as this is what you are really protecting allow for a full load current of 150% on startup I personally would use a 4 amp breaker to avoid intermittent tripping in power on situations.

 

[BryanG]

The supply could easily pull 20Amps on a cold start, so make sure you pick a type that can cope with that inrush. The thing to remember is that you can't protect the power supply from a fault, the protection is there for the wiring to the power supply and to limit any blue smoke coming out of the supply. If the power supply has a fault it is already an ex-power supply.

 

[Scottryan03]

I always follow mfg recommended circuit protection. Technical data under circuit protection

Power supply (2687-2142) | WAGO USA

Power supply; Eco 2; 1-phase; 24 VDC output voltage; 1.25 A output current; DC-OK LED (4066966156492) Online at WAGO USA!

www.wago.com

 

[JesperMP]

The supply could easily pull 20Amps on a cold start,

The old style PSUs with transformer, rectifier bridge and bank of capacitors did have a very high inrush. Modern day switched mode PSUs not so much.

I always follow mfg recommended circuit protection

+1
That being said, I cannot find the exact information in the Wago manual for the recommended protection on the input side.
It does say that the typical current is 0.6A and inrush current can be up to 10A.
The PSU is shortcircuit protected on the output side. You should only need to consider to protect against short circuits from the 110V power supply to the PSU.
So to put it short I would select a 2A type C circuit breaker.

 

[Scottryan03]

 

[JesperMP]

Ah, it was on the website but not in the manual.

 

[BryanG]

This is the inrush for a Meanwell 30watt switch mode power supply.

 

[JesperMP]

This is the inrush for a Meanwell 30watt switch mode power supply.

I cannot explain but today I do not experience the problem with switch mode PSUs tripping the circuit breakers (*) in the same way that the old style PSU's would do.
Maybe because the time that the inrush current lasts is much shorter ? I know that the short circuit trip in circuit breakers is in principle 'instantaneous' but maybe there still needs to be a certain time however short for there to be enough energy to trip the breaker ?

*: Unless you took measures such as selecting type D and/or significantly over-dimensioning the protection in front of the PSU.