Transformer secondary protection

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Grouch1980

Grouch1980

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Using Table 450.3(B) for transformer protection, let's assume I fall under the 2nd row, where primary and secondary protection are both needed. What is the reasoning for allowing up to a max of 125% for secondary protection? (or next standard size higher allowed by Note #1). I know transformers are typically 100% rated. Does the 125% max rating exist to compensate for continuous loads, just in case they are used?
 
Grouch1980 said:
Using Table 450.3(B) for transformer protection, let's assume I fall under the 2nd row, where primary and secondary protection are both needed. What is the reasoning for allowing up to a max of 125% for secondary protection? (or next standard size higher allowed by Note #1). I know transformers are typically 100% rated. Does the 125% max rating exist to compensate for continuous loads, just in case they are used?
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450.3(B) is about overcurrent protection settings (1000V or less). The reason for the code-makers stipulating a 125% setting is that circuit protection devices are tested in the open air (not enclosed) and there are cooling effects in the test environment that are not present when these devices are utilized (inside cabinets).
However, there is a code provision that mentioned a 125% minimum transformer sizing requirement when the transformer is supplying fire pumps:

695.5 Transformers. Where the service or system voltage is different from the utilization voltage of the fire pump motor, transformer(s) protected by disconnecting means and overcurrent protective devices shall be permitted to be installed between the system supply and the fire pump controller in accordance with 695.5(A) and (B), or with (C). Only transformers covered in 695.5(C) shall be permitted to supply loads not directly associated with the fire pump system.​
​
(A) Size. Where a transformer supplies an electric motor-driven fire pump, it shall be rated at a minimum of 125 percent of the sum of the fire pump motor(s) and pressure maintenance pump(s) motor loads, and 100 percent of the associated fire pump accessory equipment supplied by the transformer.​
Hope that helps.
 
Nice. You always learn new things here!

So 2 follow up questions….
1) how come this 125% max rule doesn’t apply to regular feeder breakers, or branch circuit breakers (I’m familiar with the 125% requirement for continuous loads, but I think this is different). Does it have to do with the heat output of transformers that could affect the operation of the secondary breaker? Or even the primary protection as well?
2) why don’t they test the breakers inside enclosures as well? How come only open air?
 
I beleive that the inrush would only affect the primary. Are you even required to use the 125% factor for the secondary?
 
infinity said:
I beleive that the inrush would only affect the primary. Are you even required to use the 125% factor for the secondary?
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In rush does only affect the primary, and it is independent of secondary loading. In reality the secondary device can be any rating.

The 125% OCPD on the secondary is a maximum possible value only for selecting the primary protection value. The primary OCPD must not exceed 125% unless the secondary is chosen not to exceed 125%.
 
infinity said:
I beleive that the inrush would only affect the primary. Are you even required to use the 125% factor for the secondary?
Click to expand...
How does that work? If you have inrush on the primary, it doesn't translate onto the secondary? I'm watching a few videos on the magnetic flux in a transformer, the B-H curve, and the resulting inrush current. I didn't know the inrush is only on the primary. Trying to put this all together.
 
Is this the reason? When a transformer is powered, the magnetic flux in the core can initially be very high, since it's out of phase with the applied voltage on the primary. This causes you to be on the 'saturated' area of the B-H curve... where it causes high inrush current. But from an article I found: "When a transformer is “saturated” it loses its ability to transfer energy to the secondary effectively, usually causing excess heating"

Does this explain why secondary current is not affected?
 
Grouch1980 said:
Is this the reason? When a transformer is powered, the magnetic flux in the core can initially be very high, since it's out of phase with the applied voltage on the primary. This causes you to be on the 'saturated' area of the B-H curve... where it causes high inrush current. But from an article I found: "When a transformer is “saturated” it loses its ability to transfer energy to the secondary effectively, usually causing excess heating"
Click to expand...

Grouch1980 said:
Does this explain why secondary current is not affected?
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Basically inrush establishes the magnetic field. The transformer action, producing current on the secondary, does not really occur until the magnetic field has been properly established and is not saturated.
 
jim dungar said:
Basically inrush establishes the magnetic field. The transformer action, producing current on the secondary, does not really occur until the magnetic field has been properly established and is not saturated.
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ah ok. I follow that part.

and going back to my original question, about sizing the secondary protection at 125% max, is the reason because the transformer produces a lot of heat, thereby allowing you to size the OCPD at 125%? I'm assuming post #2 by Topgone is implying too much heat from the transformer would affect the OCPD.
 
Grouch1980 said:
... going back to my original question, about sizing the secondary protection at 125% max, is the reason because the transformer produces a lot of heat, thereby allowing you to size the OCPD at 125%? I'm assuming post #2 by Topgone is implying too much heat from the transformer would affect the OCPD.
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The heat produced by the transformer would not seen any protective device unless the device was mounted in/on the transformer.

As I have stated in other threads, the 125% value probably has no scientific basis and was likely added as a fudge factor which products and testing have been designed to for almost 100 years.
 
Transformer rating is a continuous rating, yes? So a limit of 100% OCPD on the secondary would require the use of a 100%-rated OCPD to be able to use the full transformer capacity for continuous loads. A 125% limit allows the use of regular OCPD for that loading case.

Cheers, Wayne
 
wwhitney said:
Transformer rating is a continuous rating, yes? So a limit of 100% OCPD on the secondary would require the use of a 100%-rated OCPD to be able to use the full transformer capacity for continuous loads. A 125% limit allows the use of regular OCPD for that loading case.

Cheers, Wayne
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Yes, transformer ratings are continuous. (so I've read in different places). That makes sense... using a 125% sized OCPD to accommodate the continuous loads.
 
ok, so I'm confused with this part actually. Some articles angle the reasoning for high inrush current differently.

Some articles say high inrush current is caused by the primary coil acting as a short circuit when the transformer is first powered... eventually the inductive reactance builds up and lowers the current to a normal steady state value.

Other articles say the high inrush current is caused by the magnetic flux of the core being in the saturated part of the BH curve, and at this point the current value will be extremely high. So the flux will cause the inrush current.

Which is it? or are these 2 connected somehow?
 
sorry... EDIT: Other articles say the high inrush current is caused by the magnetic flux of the core needing that high current, since we're in the saturated part of the BH curve, and at this point the current value will be extremely high. So the high inrush current is needed to produce that high flux value.
 
I do have one more question... the inrush current needed on the primary side... is that symmetrical current or asymetrical current around the x-axis (assymetrical thus requiring a dc offset)?
 
Grouch1980 said:
I do have one more question... the inrush current needed on the primary side... is that symmetrical current or asymetrical current around the x-axis (assymetrical thus requiring a dc offset)?
Click to expand...
Faults and startup inrush will have DC offset
 
Grouch1980 said:
Here's actually a good article on the different scenarios a transformer faces when switched on... and how inrush current is affected by different voltage scenarios:

if you scroll down, you'll see scenarios A through F.
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Based on the linked article... transformer inrush current is only a DC offset... that's what i gathered from all the examples in the article. or does inrush current for the transformer also include a short circuit symmetrical component? it looks like it's only a DC offset.
 
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