Article 240-I

[billw]

I am writing to ask about the overcurrent protection that is described in Section 240-I. First, I have read through the sections relating to overcurrent protection for cables rated 600V or less. I have noticed that the wording seems much more specific regarding what is allowed for protecting cables in the 600V or less category. In contrast, the wording for overcurrent protection of cables in the over 600V category seems much less stringent.

Article 240-100 (b) simply states that the overcurrent protective devices shall be capable of detecting and interrupting all values of current in excess of their trip settings. This article does not seem to provide any limitations on what can be set.

Article 240-100 (c) states that the overcurrent protetive devices must protect agianst excessive temperatures caused by short-circuit currents.

Article 240-101 (a) allows fuses to be sized up to three times the ampacity rating of the cable. The long time trip element setting of a breaker is allowed to be set up to six times the ampacity rating of the cable.

Allow me to use an example from Table 310-79. The cable is 2/0 copper, Type MV-90, with an ampacity rating of 200 amperes. Based on my understanding of Article 240-101 (a) this cable could be protected against overloads with a long time trip element setting of the circuit breaker at 1200 amperes. Is this the intent of Article 240-101 (a)? Article 240-100 (c) requires protection against damaging temperatures caused by short circuit currents, but Article 240-101 (a) seems to allow damaging tmeperatures to the cable caused by continuous load current.

Can anyone please provide some clarifications on these Articles of the NEC?

 

[rick5280]

Re: Article 240-I

Bill, IMHO I would think that, at the higher voltages that you are talking about, the time curve for any overcurrent device to open would be quicker than the time needed for the conductor to be damaged.

Remember, at this higher voltage, most overcurrent equipment should be engineered.

Maybe someone with Bussman experience can give us the background for this.

Rick Miell

 

[billw]

Re: Article 240-I

Rick,

Thanks. My concern is with a protective relay supposedly providing the overcurrent protection for a 5 kV cable. A 1,200 Amp pickup for the long time delay setting of the relay would allow 1,199 Amps continuously through a 2/0 AWG MV-90 cable rated at 200 Amps. This seems to defy all the rules applicable below 600 Volts where conductors must be protected at their ampacity.

 

[rick5280]

Re: Article 240-I

billw, why don't you visit an engineering site (www.IEEE.org or .com)? I tried looking through my copy of the Standard Handbook for Electrical Engineers, and got lost at the index.

I think this protection limit helps when there is other protective devices ahead of the fuse, as in a recloser. You want the recloser to act up to three times before locking off, so there will be substantial current flowing. This will allow a bird or other varmet to be knocked loose, and still have the power on. You want the fuse to be able to handle this current, and you also want the fuse to open before the recloser's last try, but you don't want the fuse to open in the event of a transient spike.

You might want to visit with a utility person to get this all striaght.

Maybe Bennie from the Northwest can enlighten us on this subject, as he seems to be well versed in higher volotages.

Rick

 

[charlie]

Re: Article 240-I

Think of over 600 volt installations the same as motor circuit wiring. The overload is controlled by the load you place on the wire or cable and the fuses/circuit breakers/relays will protect the wire or cable from short circuits and ground faults. Each load (transformer banks, motors, capacitor bank, etc. will have its own overcurrent protection).

Generally speaking, the protection should be designed by a qualified engineer to provide coordination of overcurrent devices and wire or cable protection.

 

[bennie]

Re: Article 240-I

I agree with Charlie. In my opinion, the NEC should defer all wiring above 600 volts to the NESC.

The provisions of the NESC are designed to keep power on. The NEC is focused on turning it off.

Outside wiring can safely operate at higher temperatures than insulated inside systems. The melt point, (withstand rating) of the conductors, outside, is a factor when determining overcurrent sizing.
The insulation integrity (withstand rating)is the factor for determining inside overcurrent sizing.

Applying NESC calculations to inside wiring will likely cause an immediate melt down of the premises.
Applying NEC rules on outside systems, will not burn down the building because the power will be off most of the time.

Both systems NESC and NEC are looking at different situations, there is a bit of overlap that can cause problems unless a good understanding of both principals of operations are understood.

An overcurrent device, being considered, short circuit protection, is not quite correct. The device will not prevent a short circuit, it will clear the shorted circuit from the system.
The overcurrent device is actually a system protector, not a circuit protector.

This is why proper co-ordination of overcurrent devices, is necessary. You can't protect the failed system, you are protecting the functioning system.

[ June 11, 2003, 02:31 PM: Message edited by: bennie ]

 

[billw]

Re: Article 240-I

Charlie & Bennie,

The work I am questioning is being performed for a large commercial facility. It involves the protection settings for a MV underground feeder within the facility. My understanding is that this is covered by the NEC and not by the NESC. The work is not part of the utility's system but part of the cutstomer wiring system.

 

[charlie]

Re: Article 240-I

Bill, what we are saying is that the process of determining the overcurrent protection changes as soon as you pass 600 volts. The Code is written more in the manner of the NESC for the over 600 volt installations. What Bennie is saying is correct (even where he corrected me ). In other words, look at the installation's protection like you are protecting a motor circuit without the overloads (you have to be aware of how much load you are putting on the circuit). The overcurrent protection will protect the rest of the installation and keep the cable from burning too much upon a ground fault. This system will keep most of the lights on.

 

[brian john]

Re: Article 240-I

Bill:

What is the Long Time Delay for the relay? I assume this is an inverse time delay relay?

 

[bennie]

Re: Article 240-I

Billw: Electricity does not care who owns the equipment, there is engineered safety in the NESC procedures. Remember the NESC is a trademark of the IEEE.

The NESC procedures apply to more than the utility projects. Some jurisdictions are recognizing this fact. Oregon State accepts the NESC for compliance on private owned systems.

Charlie: That'll be the day when I can correct you I wanted to point out how we all call an overcurrent device, short circuit protection.

Insulation is the only short circuit protection.
I call an overcurrent device "a short circuit, and fault clearing device".

 

[bennie]

Re: Article 240-I

A time over-current relay(TOC), will operate when the over-current is exceeded. The over-current is based on the withstand or melt point of the conductor. The trip is usually set at 50% of the melt point.

A No. #4 ASCR conductor will carry 500 amps for 10 seconds, and 16,000 Amps for .01 seconds. before the conductor or insulation is damaged.

The pick up current, and time, has to be coordinated to take only the faulted section off line.

 

[juanito]

Re: Article 240-I

I would like to respond to several of the comments that have been offered to BillW. First, with regards to the cable type, ACSR is an open-air conductor while MV-90 is an insulated conductor; for example EPR or XLPE. The thermal conditions for ACSR and EPR are quite different for obvious reasons. Of course, there are limitations associated with both that must be considered. I mention this only because BillW noted that he has an MV-90 cable in a commercial distribution system. Second, with regards to the definition of over-current, this is defined in Article 100 of the NEC as ?any current in excess of the rated current of the equipment or the ampacity of a conductor. It may result from overload, short circuit, or ground fault?.

From my own experience damage curves for medium voltage cables are usually well above the settings of the protective devices used for protecting these cables. The damage curves for medium voltage cables are usually an issue with multi-conductor circuits; that is, when one uses two or more cables per phase.

So, if I were to assume that BillW?s cables were protected for short circuit and ground fault currents, we would have two of the three types of over current covered. Based on the Article 240-I one could select an amp tap setting that was two to six times the ampacity rating of the cable. Though this would not provide any overload protection for the cable, my understanding from the responses is that this is not the focus of protection for cables at this voltage level. Rather, the design of the distribution system is to guarrantee that an overload condition would never occur. So, I ask ?Is this practical??.

What is so magical about cables under 600V that the code would require overload protection in this situation and not for over 600V cables? Under 600V cables must also be designed to carry the load current. This would exclude motor circuits that require other considerations for the application.

What does the protection engineer do in the situation where an exisintg system was properly designed in the beginning, but over time additional loads, in the form of transformers, were added to a 5kV system resulting in a condition where an overload of the cable was possible?

 

[bennie]

Re: Article 240-I

Hello Juanito: I'm pleased to see the participation of an obviously well informed person as yourself.

I agree with your comments, I was using the reference to ASCR cable as an illustration of withstand rating. Insulated cable does require other means of protection.

The trip point for overcurrent and fault current can be selected with a proper TOC sensor and relay. The sensor will not permit continuous overcurrent or short circuit current.

In some cases the trip time is to permit a fault to clear itself or clear and trip allowing the recloser to function.

 

[bennie]

Re: Article 240-I

Current control in the primary distribution system is entirely different than on the low side.

Power surges on the primary are a fact of life. The TOC relay must not react to the surge, which is usually only for a few cycles. Some TOCs are calibrated in number of cycles, others in seconds.

The low side, with internal logic breakers, does not react to power surges.

 

[charlie]

Re: Article 240-I

What is so magical about cables under 600V that the code would require overload protection in this situation and not for over 600V cables?

The skill level of the people who work on the system is normally vastly different. MV & HV systems are also normally designed by a qualified engineer.

 

[iwire]

Re: Article 240-I

Bennie and Charlie, thank you very much for this information I have learned a lot.

I never new what rules the POCO worked under, looking at it from the prospective of a under 600 volt guy it always seemed they had no rules as to overcurrent on conductors.

I must admit I felt a little insulted when I read this

The skill level of the people who work on the system is normally vastly different.

When I put my ego aside I realized this is very true.

While anyone can run down to the big orange box and try to install a service, at present they do not sell HV cutouts.

I work at facilities that own their own HV distribution and all these places have in house engineers that oversee any additions and modifications.

Bennie your comment,

The provisions of the NESC are designed to keep power on. The NEC is focused on turning it off.

Made it clear to me about the differences.

So thanks to both of you

Bob

[ June 13, 2003, 06:14 AM: Message edited by: iwire ]

 

[charlie]

Re: Article 240-I

Bob, I apologize for the selection of verbiage. "The skill level of the people who work on the system is normally vastly different." is a good statement if it is extended to indicate that the skills are in different areas. That is to say that the normal electrician can not do over 600 volt work and meet the Code. Conversely, the normal lineman can not do 600 volt and under inside wiring and meet the Code. The skill sets are different and it is a rare person that possesses both.

Sometime I get going without taking into consideration how my verbiage is coming out.

 

[iwire]

Re: Article 240-I

No apology necessary, once I thought about what you where trying to say it made perfect sense.

Your posts made me understand that there is a method that is followed.

As a utility guy can you answer a question I have, I often did service changes on apartment buildings in the city, these would typically have POCO feeds of direct buried lead cable assemblies in size 6 AWG or 4 AWG.

Now I come along and put in an 800 amp service and hook on to these 6 AWG or 4 AWG cables.

Is there a reason that the POCO does not have to upgrade, from my end this is very odd?

 

[charlie]

Re: Article 240-I

Bob, this really deserves to be on another thread because I think my answer will generate a bit of discussion.

The short answer is because we can!

Now the real answer. The load profile in dwelling units is such that there is a minimal load until the occupants get up (usually at different times) and start up the shower, coffee pot, maybe breakfast, etc. Then the load drops off as people go to work, settle down to answer e-mail, go shopping, walk the dog, etc. A small bump in load happens around noon and then a big one at dinner (I still call it supper) that tapers off until bed time when load drops to a minimal amount again.

We (at IPL) estimate the maximum load at approximately 40% of the switch size since most electrical contractors will calculate the load in accordance with Article 220 and go up to the next standard size. I don't work in our downtown area so I don't know how heavily we work the PILC wire but I assume (I know, it's a bad word) we will load it more than the NEC plus we will use the 90 degree C ampacity for rating it.

With all that said, IPL would probably not use anything smaller than 2/0 PILC for a new service. Since I do not know the parameters of your installation and the reasoning behind the power company's decision, I can't really comment on their decision. The question I have is . . . has the power company cable burned up?

 

[iwire]

Re: Article 240-I

Yeah my question was a "thread jack" but thanks for both answers

To answer your question No, never had a melt down that I know of and I did a couple dozen of these installations.

Bob