SCCR , power supply & SPD questions for an industrial control panel - detailed

mhismail

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Mar 2016
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Hi all,

we are assembling an industrial control panel which consists of 4 branch circuits supplied from panel feeder as follows:

Panel feeder : #12 AWG , which is connected directly to fused-distribution block (i will call it FDB) (200KA , 1 in - 8 out ) no other devices are connected to the feeder (no breaker , no disconnects , and so on).

(1) Branch 1 (Main circuit) : supplied from the above FDB , then Fused disconnect switch then power supply (3PH-in , 24vdc-out,5A) then supplementary breaker number 1 (3A) then PLC & I/O

(2) Branch 2 (Heater) : supplied from the above FDB , then Fused disconnect switch then power supply (3PH-in , 24vdc-out,20A) then supplementary breaker number 2 (20A) then Heater

(3) Branch 3 (Lighting) : supplied from the above FDB , then Fused disconnect switch then power supply (1PH-in , 24vdc-out , 3A) then supplementary breaker number 3 (2A) then LED

(4) Branch 4 : supplied from the above FDB , then Fused disconnect switch then Receptacle outlet

My questions:
----------------
(Q1) : At first , all the above " Fused disconnect switches " were circuit breaker , but since max interrupt capacity we found was 10 kA , that was a small value since it always lowers the global SCCR of the panel , so we thought to replace the breakers with Fused disconnect switches which dramatically increased the SCCR of all branches upstream all the power supplies at almost small acceptable cost. Now , what about the breakers downstream the power supplies ,

(a) Branch 1 : the is control circuit class 1 fed by power supply (which is similar to control transformer rules - i guess ) hence the supplementary breaker number 1 will be neglected , is that correct ?

(b) Branch 2 : the supplementary breaker number 2 has interrupt capacity of 7.5 kA and since the power supply is less than 5 kVA and less than 120 v secondary output (i will treat it as power transformer - can i do that ?) then the primary protector of the power supply applies its SCCR to the entire branch circuit hence it is ok to use the supplementary breaker number 2 , is that correct ?

(c) Branch 3 : same as Branch 2 , is that correct ?

OR , should we take all supplementary breakers into account while estimating SCCR of the panel because a power supply cannot be treated as a power transformer nor a control transformer ?

(Q2) If it is required to install SPD for the panel feeder , it will be another branch (number 5) in parallel to the above branches and supplied from FDB , is that correct ?

Thank you all for helping
 
Nothing down stream if the power supplies counts. SCCR only relates to the primary input power of the panel. Supplementary breakers between the fuses and the power supply primaries will need to have a series SCCR listing with those fuses, otherwise the entire panel SCCR becomes that of the supplementary breakers.
 
If you want a higher SCCR, you can look at a motor protective device for the main breaker on each branch. Many carry a 65k SCCR.

Fuses are even better, most bussmans will be 200k. I like the TCF fuses with the on/off switch base. You can turn the circuits on and off and still get the advantage of having a fused rating. Plus, they are finger safe.
 
Nothing down stream if the power supplies counts. SCCR only relates to the primary input power of the panel. Supplementary breakers between the fuses and the power supply primaries will need to have a series SCCR listing with those fuses, otherwise the entire panel SCCR becomes that of the supplementary breakers.

OK , I agree in general , but can I ask why?

my main problem is that I can't categorize power supplies with 24 vdc output. Most standards consider "transformers" , Siemens papers some times mention power supplies , but in general i have a problem with that.
my questions are :
(1) Is a power supply treated as control transformer if it is used in control circuits & power transformer if used for power ?

(2) Why didn't you consider the heater circuit (upstream & downstream power supply) as power branch ? is there some limiting VA to call a circuit a power one? The downstream of the power supply is class 1 power-limited circuit (Art. 725) since it is less than 30 v & 1000 VA , correct?

(3) for example a power supply 3ph-380 vac input and 24 vdc-20A output , what is the category of that power supply? Is it a class 1 circuit power-limited source?

Thank you for reply
 
Last edited:
If you want a higher SCCR, you can look at a motor protective device for the main breaker on each branch. Many carry a 65k SCCR.

Fuses are even better, most bussmans will be 200k. I like the TCF fuses with the on/off switch base. You can turn the circuits on and off and still get the advantage of having a fused rating. Plus, they are finger safe.

you are correct , that's why we replaced breakers with disconnect fused switch , but my question is : what is the role of the supplementary breakers (downstream power supplies) in estimating SCCR ?
 
you are correct , that's why we replaced breakers with disconnect fused switch , but my question is : what is the role of the supplementary breakers (downstream power supplies) in estimating SCCR ?
If the supplemntary breakers are on the load side of the power supply, they do not count in the assessment of SCCR. SCCR evaluation ONLY applies to the line side Power components. Power supplies are specifically exempt and therefore everything down stream of them are not part of the process.
 

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