Protection for Transformer Secondary conductors

[kingpb]

Lets be careful here. 240.21 has nothing to do with OCPD for the transfomer. Those rules are found in 450.3. The rules in 240.21 only apply to the conductors and the rules in 450.3 only apply to the transformer. The installer must satisify both sets of rules.
Don

Excellent point, Don! I think this was being over looked.

 

[jkim780]

Examine what you mean when you say, "fault". Bad insulation turns into a fault. A bad connection turns into a fault. If your definition of "fault" is anywhere near mine, then faulted conductors are finished - there is nothing in good working order to save.

When I said "fault", I meant "short circuit or ground fault".

Lets start with 90.1(A), (B), (C). The NEC readily admits, it is not absolute safety, it is not necessarily efficient, it is not a design guide.

One of our posters made a particularly insightful comment - I'll paraphrase, "Saying that an installation is built per code means that if it were one bit worse, it would be illegal." One of my comments was, "No I didn't build it per code, I couldn't stand that level of un-reliability."

NEC is minimums - Might be okay for your house - generally it is way too light for industrial. For example, a recent poster was looking for advice on replacing burned up conductors on a 2MW (?) gen. He was really looking to minimize the copper because the cost of copper is high. I never got a chance to answer. I wanted to advise him, that minimums were not a good idea - minimums burned up once.

Now let's take a look at 240.21 tap rules and 450.3 xfmr OCP. Especially see 450.3, FPN 1 (NEC 2002). This stuff is some serious power, commercial buildings, industrial facilities, process power and control.

IMO when you are dealing with designs and installations concerned with tap rules and xfmrs, it is up to the designer to make it safe and reliable. The NEC won't save you or make you safe.

My design and build philosophy is to:
1. Make it so it will do the intended job.
2. Make it safe for the intended user.
3. Keep it contained so it doesn't crap up the environment.
4. Make it reliable to keep the life cycle costs down.

My theory is that if you do those four things, you are well past code.

The NEC is not a design guide. The AHJs are not designers. One size does not fit all installations. When an installation gets to this level (tap rules and xfmrs), the customer and the designer figure out what is important.

I am fully aware that NEC is not a design guide. This is not the point here.

For example, you started with conductor protection and faults. Then you switched to ground fault and personnel protection. Well what is the application? Commercial highrise, or industrial process? One general answer does not fit all.

Isn't OCPD for overcurrent protection of equipment and condutors and eventually for protection of life and propety?

coulter,

I think you are missing my whole point. Please read my previous posts. I think I repeated it at least 3 times.

 

[jkim780]

jkim,

Lets be careful here. 240.21 has nothing to do with OCPD for the transfomer. Those rules are found in 450.3. The rules in 240.21 only apply to the conductors and the rules in 450.3 only apply to the transformer. The installer must satisify both sets of rules.
Don

I think that is what I have been saying all day.

 

[jkim780]

jkim,

There is no conflict. 240.21(C) provides the requirements to protect the secondary conductors where they are not protected by the primary OCPD. Yes there is some length of unprotected conductors, but according to the code the risk is acceptable.
Don

Thanks, Don. That is exactly my point. Now I see that 240.21(C) is not conflict with 240.4(F). Then My question is why does the code think that perticular risk is aceptable? what's the reason behind it? Myabe the reason that jtester pointed out?

And my next question was if my following assumption was right or wrong:

If you have a circuit breaker protecting the xfmr primary conductor (sized 125% or next higher standard rating), that circuit breaker can also provide protection for the xfmr (primary & secondary winding per table 450.3(B) primary only protection) and the xfmr secondary conductors.

 

[jkim780]

Three issues here - you have to look at all.

1. "...the primary circuit breaker can't protect secondary conductors..." Add "from overload". Yes that is true.

Also true are:

2. The primary circuit breaker will protect the personnel and equipment from secondary conductor fault.

I see that now.

3. The secondary OCPD will protect the secondary conductors, the transformer, and the primary conductors from overload and from faults down stream from the sec OCPD.

Did you mean "the OCPD supplied by the xfmr secondary conductors" ? Then, yes I see that now.

As for the length of the tap conductors, IMO the code panels used an ouija(sp) board or dart board to pick the numbers. Do I think the numbers are bad? No - not good, not bad, just are. The selections are okay - they work and are not onerous(sp)

I didn't think those numbers were bad. I was just wondering if there was any reason behind it.

 

[jkim780]

Don -
Other than 90.1, where does the code say that?

I am curious, too. Don.

 

[jkim780]

I think I found out what I was confused about.

240.21(C) states:

"Each set of conductors feeding separte loads shall be permitted to be connected to a transformer secondary, without overcurrent protection at the secondary,....."

Shouldn't be more clear if it states:

"...without an overcurrent device at the secondary...."

like coulter pointed out the secondary conductor has a short circuit protection from the primary OCPD and the overload protection from the OCPD supplied by the secondary conductors. and if , in deed, that is true, the secondary conductors are not "unprotected conductors" after all. Are they?

 

[coulter]

I am curious, too.

At first I was wondering where the NEC said the risk to the sec conductors was acceptable. Then I got to wondering, "What risk?"

Standard Answer: Well the conductors are not protected from overload at the supply end. Yes that is true, but any current going into the xfmr end on the sec conductors is coming out the OCPD end - unless there is a backhoe tooth stuck in them. A supply end OCPD won't save the sec conductors any more than a load end OCPD.

So, the risk appears to be mechanical damage. How do you save conductors from that? Steel and concrete is the best I know of.

Conclusion:
Mitigation of risk to the sec conductors is not dependent on the position of the OCPD, but rather by the amount of steel and concrete guarding the conductors.

So, now I'm asking, "What risk?"

And, in Don's defence, I would have to say the NEC implies acceptable risk in 90.1, and by conspicous absence of regulation. For example, wouldn't gfi's on all ckts be safer. (maybe it would, maybe it wouldn't matter - this is an example). Apparently the NEC thinks it is acceptable risk to not have them in all locations.

carl

 

[don_resqcapt19]

Don -
Other than 90.1, where does the code say that?

Just the fact that there is a rule that sets a length means that the proposal writer thought that the risk was acceptable and the CMP must have agreed to accept the proposal.
Don