120VAC to 24VDC Circuit Breaker Help

[Iawia]

Hi All,

I have a system of PLCs (which control a motor) that I am trying to protect from overcurrent / power surge. The associated motor runs on an 75VDC and has a conversion stage from 120VAC to 75VDC. The PLC CPU runs on 24 VDC so it also goes through a conversion from 120VAC to 24VDC. I am confused on where to put the circuit breaker for the PLC CPU on the 120VAC side or the 24VDC side.

The TDK Lambda converter says "24VDC/1.25A" on its face so I would assume those amps are derived from the 24VDC source? How many Amps AC would it be consuming? Does it matter which side the breaker is placed on (see image for locations 1 and 2)? Similar question for the motor as well, however I'm not as concerned since it the motor's internal converter is very advanced and has a lot of protection and will shut down as soon as it senses any problems.

The 24V TDK rectifier specs are here (DRB30-24-1).
A proposed 1A breaker for protecting the CPU (voltage rating says "240VAC / 48VDC, 1A"?

Or perhaps, both locations are appropriate? I just want to make sure I'm implementing this right, or if I am overthinking (I'm not an EE) ... Is protection even needed? Maybe that is also an appropriate question.

Thanks guys.

 

[Diver300]

Fuses won't deal with surges. They can only protect against overcurrent.

A breaker at 1 would only protect the wire between it and the 120 V to 24 V converter, but an overcurrent there would only be caused by the 120 V to 24 V converter failing.

A breaker at 2 should be a higher rating than 1 A. It probably wouldn't be necessary as any well-made 120 V to 24 V converter will be current-limited. The 24 V output voltage will be reduced without damage if there is a short circuit.

 

[rjenkinsgb]

Fuses and breakers are generally to protect wires from overload and prevent overheating or fires.
Fuses or breakers should be used at any point a high current power source splits to lower current wiring.

You can also add fuses for different sections of a PLC systems, eg. one each for the PLC itself, one for local inputs, one for distant inputs, one for local outputs one for distant outputs etc etc.
They can make future faultfinding much easier.
(By local, I mean the operator panel or control panel the PLC is in, distant being switches and sensors around the machine being controlled).

AC power Surge protection is done using purpose made filters or VDR based modules.

Control and signal line protection is by proper wiring layout and screening where appropriate, keeping all low voltage wiring physically separated from power wiring (eg. separate steel trunking for low voltage and power cables) and proper suppression of all inductive loads, both such as relay and contactor coils and AC loads such as motors.

Cables from high frequency inverters / drives to motors should also be screened.

 

[Iawia]

Thank you rjenkinsgb & Diver300 for your thoughtful responses. RJ I do get you on the local screenings and proper layout, I guess I am still unclear about where to put the breaker (location 1 or 2) and the reasoning behind either. If the only reason would be to protect the wring I can just leave it out since wiring is all oversized anyway. My original thought was to protect the CPU and PLC hardware.

But just for argument sake, are locations 1 & 2 both pulling the same current? One side is AC and the other DC, I am just trying to wrap my brain around that. If this particular inverter needed protection, what would be the reasons for putting at location 1 or 2?

Thanks in advance!

 

[Nigel Goodwin]

Thank you rjenkinsgb & Diver300 for your thoughtful responses. RJ I do get you on the local screenings and proper layout, I guess I am still unclear about where to put the breaker (location 1 or 2) and the reasoning behind either. If the only reason would be to protect the wring I can just leave it out since wiring is all oversized anyway. My original thought was to protect the CPU and PLC hardware.

That's not what fuses do, and never has been. A fuse protects for over current - if point 2 has over current, then there's already a serious failure in the CPU/PLC section - and the fuse (hopefully) would blow in order to protect the 120/24V power supply stage. A fuse at point 1 would protect the incoming 120V supply and wiring if the 120/24V power supply died.

The point isn't to 'protect' the wiring, it's to try and prevent the wiring over heating and catching on fire.

But just for argument sake, are locations 1 & 2 both pulling the same current? One side is AC and the other DC, I am just trying to wrap my brain around that. If this particular inverter needed protection, what would be the reasons for putting at location 1 or 2?

Thanks in advance!

No, 2 is roughly five times the current of 1