Why do we need Secondary protection for the Transformer

[whistler]

Hi All,
This might be a really stupid question but I am trying to find why is there a secondary protection for a transformer? The reason I am confused is because the secondary windings is getting its power from the primary winding and if the device on the secondary side pulls more current wouldn't that mean that more current is being drawn from the primary and the breaker on the primary side would trip.

Thanks,

 

[Pierre C Belarge]

Hi All,
This might be a really stupid question but I am trying to find why is there a secondary protection for a transformer? The reason I am confused is because the secondary windings is getting its power from the primary winding and if the device on the secondary side pulls more current wouldn't that mean that more current is being drawn from the primary and the breaker on the primary side would trip.

Thanks,

That would depend on the protection scheme that is located on the primary side.
Some of the schemes allow for the primary winding protection to protect the secondary windings.

Most times when there is secondary protection, it is not for the transformer windings, it is for the secondary conductors.

 

[KevinVost]

Art 450 does not require secondary protection on a transformer see table 450.3(B). However your secondary conductors are taps and must comply with 240.21 (C), which provides requirements for conductors installed in this manor. The code requires all taps be protected, either by calculations or by an OCD.

 

[cadpoint]

Your Answer is really in the begining's of 450 but 450.5(2) Overcurrent Protection. (maybe I'm limited my conversaion stating (2) as my answer)

Now, in the Layman terms, think of a a trannie as a thing where the power is transformed, not always down-in size, either.
Thus you can consider it a trannie a new service, it has it's own bonding and grounding requirements of applying this type of service.

To compare:

What do we do first thing with Electrical Service? Apply a breaker of some sorts to it! Well with the trannie your first protection is this means of disconnector/breaker in respects to the downline service, much like what we do everyday to protect a circuit or service without a trannie envolved....

Welcome to the Forum, The only bad questioin is the one not asked!

 

[anbm]

If the transformer feeds a panel, look at 408.16 for panel protection.

 

[macmikeman]

Following all the rules exactly for protecting the primary conductors and not having to protect the secondary conductors has for me resulted in a zero pass rate from the building dept on both the times I attempted to utilize the method.(with the blessings and permission of the EE who designed the installation without secondary protection.) Both times the local inspectors told me flat out-" Mike, we are helping you out here, from a safety standpoint by not letting this fly. At the time I didn't like the call, but lots of years passed since then, and now I think maybe it wasn't such a bad deal for me after all to loose a few bucks buying extra equipment. I never had to deal with a fire on one of my projects from a tranny, but I know some guys who did. Glad it was them and not me.

 

[don_resqcapt19]

Following all the rules exactly for protecting the primary conductors and not having to protect the secondary conductors has for me resulted in a zero pass rate from the building dept on both the times I attempted to utilize the method. ...

Unless you have a two wire to two wire transformer or a 3 wire delta to 3 wire delta transformer, there is no code rule that permits the primary OCPD to protect the secondary conductors.

 

[macmikeman]

Unless you have a two wire to two wire transformer or a 3 wire delta to 3 wire delta transformer, there is no code rule that permits the primary OCPD to protect the secondary conductors.

My mistake, I had this reversed. In and around 1990 I tried twice to use the allowances to not have a primary overcurrent device, and instead rely on the primary feeder overcurrent device exceptions, not the secondary side. It still didn't pass go regardless.

 

[donw]

Unless you have a two wire to two wire transformer or a 3 wire delta to 3 wire delta transformer, there is no code rule that permits the primary OCPD to protect the secondary conductors.

Don, for supervised industrial installations, what about 240.90(D)?

 

[Rockyd]

Not to be a art. police guy.... but it's 408.36 for the panelboard OCPD protection issue.

As Kevin said - 90% of it is just walking through 240.21(C) 1 through 6.

 

[don_resqcapt19]

Don, for supervised industrial installations, what about 240.90(D)?

Good point. I forgot about that "exception".

 

[Besoeker]

Hi All,
This might be a really stupid question but I am trying to find why is there a secondary protection for a transformer? The reason I am confused is because the secondary windings is getting its power from the primary winding and if the device on the secondary side pulls more current wouldn't that mean that more current is being drawn from the primary and the breaker on the primary side would trip.

Thanks,

To prevent this kind of thing happening maybe?

 

[Goroon]

3 phase transformer 3ph load

3 phase transformer 3ph load

Just a Thought....
240.4 Protection of Conductors.
(F) Transformer Secondary Conductors.
Single-phase (other than 2-wire) and multiphase (other than delta-delta, 3-wire) transformer secondary conductors shall not be considered to be protected by the primary overcurrent protective device...

I look at Turns Ratio for the Primary to Secondary Current.
Formula for both 2 wire and 3 wire delta where Loads are balanced.

If this is [example] a Delta-Wye, unbalanced, a line current of 100A on any secondary, Unbalanced X1 to X0 or X1 to X3 would create 43.3 amps on the Primary Delta. With out secondary protection and primary @ 125% allowed to next standard size, what size do yo size the Secondary conductors if Not Protected?

 

[shahnawazbachh]

some time we are using secondary protection, in case of primary protection fail, it works as a back up protection.

 

[Pierre C Belarge]

Protecting transformer windings [Art 450] and conductors on the primary and secondary of transformers [Art 240] are two distinctly different Articles and functions.
The one item that sometimes confuses some, is that the protection provide by Article 450 sometimes will overlap the protection required by Article 240.

 

[LarryFine]

Single-phase (other than 2-wire) and multiphase (other than delta-delta, 3-wire) transformer secondary conductors shall not be considered to be protected by the primary overcurrent protective device...

This is because only these two types directly reflect the secondary current in the primary. The rest can be partly overloaded because more than one primnary section or segment contributes to one secondary section's or segment's current, or vice versa.