Three Related Questions: #2

[charlie b]

Common information for all three: Transformer within outdoor substation. Cables connect secondary to busbars. Breakers serve loads both inside and outside the adjacent building. Primary is over 600 volts, secondary is 120/208V.

Question:
Table 450.3(A) allows us to double the rating of the secondary protection for supervised locations. Why? If you overload something, it is going to fail, so what difference does supervision make? And how does the qualification of a future maintenance worker affect the performance of a transformer that is, here and now, being overloaded? What I really want to know is how do I take advantage of the 250% factor? Do I have to install sensors, so that a ?qualified person? has the ability to ?monitor? the transformer?

 

[steve66]

Common information for all three: Transformer within outdoor substation. Cables connect secondary to busbars. Breakers serve loads both inside and outside the adjacent building. Primary is over 600 volts, secondary is 120/208V.

Question:
Table 450.3(A) allows us to double the rating of the secondary protection for supervised locations. Why? If you overload something, it is going to fail, so what difference does supervision make? And how does the qualification of a future maintenance worker affect the performance of a transformer that is, here and now, being overloaded? What I really want to know is how do I take advantage of the 250% factor? Do I have to install sensors, so that a ?qualified person? has the ability to ?monitor? the transformer?

I think the intent is that the load calculation limits the actual load on the xformer. The larger breaker just allows fewer false trips due to things like motor starts, etc.

Supposedly, in a supervised location, someone would verify the load calculation before adding more loads.

That's what I've always assumed was the intent was.

 

[dana1028]

Common information for all three: Transformer within outdoor substation. Cables connect secondary to busbars. Breakers serve loads both inside and outside the adjacent building. Primary is over 600 volts, secondary is 120/208V.

Question:
What I really want to know is how do I take advantage of the 250% factor? Do I have to install sensors, so that a ?qualified person? has the ability to ?monitor? the transformer?

The 'qualified person' must have ESP so that he can 'sense' trouble before it occurs thus preventing problems. These people are readily available as advertised in the "Psychic Journal.":angel:

 

[hurk27]

Most loads down stream of a transformer will have over load protection, except maybe a fire pump which is design to run till it smokes, but the fact is if the system is design correctly the down stream OL's or OCPD should prevent such an over load on these conductors, but it is true that new loads can get added, so the supervising must be in place also when adding loads to an existing system.

 

[chris kennedy]

Here is the HB commentary FWIW.

For Note 3 of Table 450.3(A), a supervised location is where maintenance of the equipment is performed by personnel who have received safety training and are familiar with proper operation of the equipment and aware of the hazards associated with it. See Article 100 for the definition of a qualified person.

 

[kingpb]

I know I've stated this before, but I will offer it again.

You cannot blindly use the NEC maximum allowable setting of the protective device ahead of a transformer without considering the thermal damage curve of the transformer. In most cases, operating at the maximum allowable is above the damage curve of the transformer. This does not change in a supervised, or unsupervised location.

Frankly, I don't know what engineering was done to come up with this basis for protection. The cable between the OCP and the transformer, if sized properly is not an issue as cable has a much higher thermal damage capability compared to a transformer.

The difference between supervised and unsupervised to me, is that in the supervised location, changes will be made halfway knowing the outcome/consequences. In an unsupervised location the same changes will be made, except they probably aren't aware of the consequences. Either way the results are the same.:slaphead:

 

[mbeatty]

I think the intent is that the load calculation limits the actual load on the xformer. The larger breaker just allows fewer false trips due to things like motor starts, etc.

Supposedly, in a supervised location, someone would verify the load calculation before adding more loads.

That's what I've always assumed was the intent was.

I agree. Too often, I see transformers that were sized without taking high inrush and/or future loads into consideration.

 

[petersonra]

i think the intent is to allow places with better qualified electrical people to cheat a little more than places that do not have those kind of people around on a regular basis.

 

[JWCELECTRIC]

Charlie,

As an Engineer the code may allow you to go to 250%, but not knowing the crews that will be over seeing the operation, lower the setting on the OCPD to 125-150% if you are recommending installing one. Also you could lower fuse ratings on the primary protection.

Add a digital multi-meter on the secondary bus/cables for monitoring and send the signal out if there is Scada on-site. If not add a autodialer if the load gets over a certain point then it can send out alarm before anything trips.

Usually these Army bases have there own power companies that do this set up as common practice.