FAULTY ELECTRICAL WIRINGS - PART 3

GOING BACK TO BASICS

FAULTY ELECTRICAL WIRINGS - A CLOSER LOOK
(Third Part of a Series of Six)

By Doods A. Amora, PEE 1821
(January 2008)

4.0: THE SHORT CIRCUIT

To the layman and even in the Philippine media, it is common to misuse "short circuit" to describe any electrical malfunction, regardless of the actual problem.

To the electrical practitioner however, the term short circuit can take two forms: (1) a ‘bolted short’ where a firm metal-to-metal contact is made across a full-thickness section of metal as if they are 'spliced' or 'bolted' together; (2) an ‘arcing short’, where initial metal-to-metal contact is not sustained and current flows through an arc.

* In a ‘bolted short’, heating is not localized at the fault point but is distributed over the entire length of the circuit. A bolted short can readily be created by mis-wiring a circuit and then turning ‘on’ the circuit breaker. The circuit breaker then typically trips before anything ignites.

* Other short circuits are of arcing nature. An ‘arcing short’ results from a momentary contact of two conductors. This causes melting of the material around the contact area. Magnetic forces tend to push the conductors apart - creating sparks as the conductors come apart. After an arcing short, large-diameter conductors can often be seen with a notch on the surface; smaller diameter wires may be severed or vaporized entirely.

What’s in Short Circuit That Can Start a Fire?

The phenomenon called ‘arcing’ is the source of many fires.

Just like a matchstick, the fact that an arc flash is produced at highly elevated temperatures during a short circuit is enough to demonstrate that it could ignite a full blown fire. An arc flash occurs when there’s a fault in the wiring system and the electric current has to move through the air to complete the circuit. If there is flammable material near one of these extremely hot arcs, it can burst into flames, starting a fire.

Arcing Short Circuits may be set-up by any of the following conditions: a) Un-Workmanlike Installation, b) Substandard Electrical Products, c) Circuit Overloading Leading to Insulation Breakdown then Short Circuits, or, d) Insulation Breakdown by External Causes.

But the question is, “how then can a short circuit ignite a fire when over-current protective devices (OCPD’s) are placed there to trip off & interrupt the circuit in these eventualities?”

This observation is of course true, but not quite true all the time…huh?

Short Circuits Created by Mis-Wiring

A circuit breaker interrupting a circuit during the first energization event of an electric system can only happen when there is; in the first place, a mis-wiring that resulted to a bolted short. In this case however, there is not much concern, because most likely, there are engineers or electricians attending to the first energization of the system and problems, whatever they are, are quickly fixed.

And it’s worthy to note the words of Mr. Vytenis Babrauskas, Ph. D., (in his paper entitled “How Do Electrical Wiring Faults Lead to Structure Ignitions” presented in an international conference in London), when he said, “it is, in fact, exceedingly hard to create a fire in branch-circuit wiring from a bolted short”.

So then, there is a big difference between ‘mis-wiring’ and ‘faulty wiring’. Note that in ‘the mis-wired (shorted) circuits’, it won’t allow your circuit to be energized, because as we know, the circuit breaker trips.

On the other hand, in ‘faulty wiring’, it will. And faulty wirings will just be there all the years lurking & waiting for opportune time to go wrong given the right conditions to go wrong!

That’s why faulty wiring is more dangerous than ‘wrong wiring’. It gives people the faulty feeling that the circuits are protected. Browse the web and you’ll find out that it’s also happening to jetliners, military warplanes and even in the Space Shuttles!

Arcing Short Circuits

Several cases of Arcing Short Circuits are caused by un-workmanlike installations. Un-workmanlike installations come in many forms. Nails or screws penetrating into mechanically unprotected wires and cables beneath walls and ceilings are most common scenes..

Poor connections also trigger over-heating, then to arcing short circuit and finally ignition. If a connection is not mechanically tight; it can start to undergo a progressive failure mode. Subsequently even in tight connections, a number of instances like copper-to-aluminum or even aluminum-to-aluminum splicings develop into what they call as “glowing” connections, especially following the popularization of aluminum wiring in residential and mobile home construction in the 1970s. A glowing connection might typically be found in a wall cavity, where the tiny combustibles are close.

While nails or screws injuring the ‘unseen’ wires & cables hidden beneath ceilings or walls are un-workmanlike installations; the mechanically unprotected wires & cables are themselves, un-workmanlike. So the chain of events, all taken together; forms critical situations - leading to electrical accidents. It’s only a matter of time...

So then, a host of other events like rats eating up wire insulations, or circuit overloading leading to insulation breakdown and substandard electrical products that overheat - carbonizing live parts into arcing short circuits are just a few in the list.

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(Pictures Galore: The illustrations above show how an electrical system can progressively go wrong, given the right conditions to go wrong!)

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5) THE INTEGRITY OF DESIGN

But then, a good physical wiring is not a guarantee to a flawless operation as years go by. While the so-called mis-wiring and the quality of installations are forefronts of the master electrician’s job, the design part of the system is the engineer’s responsibility. It’s worthwhile to mention that any good electrical system starts with a good design. That means - an electric system should start from a good engineer, too.

As faults usually don’t happen weeks after energization; the probability of faults in electrical systems increases with age. Faults such as “short circuits” and “ground faults” will most likely happen, whether we like it or not – in the near or distant future. The overlapping root causes & conditions cited herein this article and their domino effects somewhere, somehow lead to faults within the lifetime of the building.

However, there are precautions that prevent faults from developing into disaster proportions. That’s supposed to be the role of electrical design engineers. In other words, what are circuit breakers for?

The OCPD Itself Creating the Fire

But then history tells us that in some events of short circuits, it was the circuit breaker itself that ‘disintegrated’ and ‘exploded’ – in the process, igniting a full-blown fire? Such, is a disgraceful irony when the protective device itself started the fire!

Is it really possible? Of course, yes! Look at the pictures below!

But why…?

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(Frame 1: A severely damaged circuit breaker after interrupting a fault. Why the damage when circuit breakers are supposedly the protection of circuits?)

(Frame 2: Worse condition. The circuit protector exploded while interrupting a fault. Why the explosion?)

(The pictures above are circuit breakers on fire in simulation tests while interrupting single-line-to-ground and short circuit faults. Note that in these cases, the OCPD’s themselves are on fire.)
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Here’s why…

THE MISUNDERSTOOD KAIC

The protective devices commonly found in electrical construction plans & drawings usually indicate only the continuous current ratings but not the interrupting capacities (expressed in KAIC) incumbent in these devices. This brings the breaker interrupting ratings as one of the most taken-for-granted components in system designing. Why is KAIC important?

A circuit breaker has three most important ratings – a continuous current rating, a voltage rating and an interrupting capacity (IC) rating. The KAIC rating is the maximum amount of current that the device will open safely to relieve a fault condition - without injuring itself.

By ‘injury’, it means the condition where after interrupting a fault, the breaker ceased to be operable - or worse, the breaker disintegrated because the fault current is too much for the breaker to handle. It must be remembered that breakers are placed in the system to protect the system itself. What if during a fault, it is the protective device that disintegrated? Isn’t it the design engineer’s lapses?

Well and good, “the installation is operating for five years and no untoward incident like this happened, if it is my fault, why only now?”, says the engineer. Fine.., because in five years time there was no fault!

To recall, circuit breakers are not placed in the system for normal conditions; breakers are expected to perform its intended work during abnormal conditions. Abnormal conditions don’t just happen during the first energization event. They always happen when the installations are going older where faults are lurking as loads are becoming more demanding.

But alas, many designers always miss sizing the OCPD interrupting capacities probably because of the usual assumption that the fault duties on the 230 v system are not necessary! This assumption is however faulty because fault duties vary largely with the size of the source transformer, the system voltage and the impedance of the cables before the points where the fault is subjecting to. For instance, if a 230 v lighting panelboard is receiving supply from a 500 KVA transformer, the three-phase fault duty at its secondary terminals could be as high as 40 kilo-amperes. If the source transformer is 1,500 KVA, the fault duty could be 70 kilo-amperes. In like manner that a 100 KVA three-phase transformer delivers a three-phase short circuit current at 10.0 kilo-amperes. These of course are dependent on the impedance of the transformers employed.

If the subject panelboards are in close proximity to the source transformer, the fault duty can be just a little lower than that at the terminals of the transformer. It is therefore important for the engineer to be aware of this reality, otherwise his design becomes faulty. The design engineer must therefore conduct fault calculations prior to specifying the IC ratings of circuit breakers no matter how small they might be. But then, are electrical engineers in this country specifying IC ratings of circuit breakers? Of course, yes! But a large number of them aren't.

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(The picture above is a Medium Voltage Circuit Breaker after interrupting a short-circuit. Note that this KAIC undersized circuit breaker disintegrated, melted and burst into flames instead of protecting the circuit. Isn’t it faulty electrical wiring?)
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Now note (just in case one may not know it), the circuit breakers commonly used in homes and offices are normally 10 KAIC rated if we talked on the usual American made GE, Westinghouse or Square D. Is 10 KAIC applicable? Maybe yes, but maybe not! It maybe correct in a house connected to a 15, 25 or 37.5 kVA utility transformers. But now in the shopping malls, we can find cheap 5 KAIC, 3.5 KAIC and some brands don’t have any KAIC rating at all. What if the house is in a high-end subdivision with a 1,000 kVA substation? Or a commercial building or condominium with a 1,500 kVA power center?

The pictures showing exploding circuit breakers in this article are presentations of the fact that circuit breakers installed in a system must possess the capability to interrupt a faulted circuit without disintegration. In other words, the circuit breaker must be sized at the correct interrupting ratings.

Now the question is, if the circuit breakers lack the KAIC capability, isn’t it faulty electrical wiring?


THE OCPD OPERATING SPEED

On top of the things discussed above, in some other cases, there are occurrences that although the circuit breaker interrupted the shorted circuit, the circuit conductor nonetheless melted, burned and vaporized, and its flames started the fire? Had it occurred to us, why?

Again, that’s the value of the competent electrical engineer in the design phase of the project.

Let us recall that the purpose of over-current protection is to open a circuit before conductors are damaged when an over-current condition exists. In fact, the OCPD’s are placed not to protect the loads, but to protect the circuit conductors. During short circuits, currents through the conductors are tremendously high that it must be removed quickly before the damage point of conductor insulation is reached.

Conductor damage points or the so-called “withstand limits” as established in the formula “i-squared x t”. The greatest damage done to components by a fault current occurs in the first half-cycle (or more precisely, “the first major loop” of the sine wave). Heating of components to very high temperatures will cause deterioration of insulation, vaporization or even explosion. Tremendous magnetic forces between conductors can crack insulators and loosen or rupture bracing structures in Panelboards, MCC’s & Switchgears.

Let me elaborate further the conductor withstand limit “i-squared x t”.

It has been established that the levels of both thermal energy and magnetic forces are proportionate to the square of current. Thermal energy is proportionate to the square of “RMS” current; maximum magnetic fields to the square of “peak” current. If a short circuit current is 100 times higher than normal load current, its increased heating effect equals (100) squared or 10,000 times higher than that of the normal current. Thus, it is extremely important, particularly since present-day distribution systems are capable of delivering high level fault currents.

Now, granting that the OCPD’s KAIC requirements are complied with. Is it enough? The answer is, NO! Because even if the circuit breaker doesn’t disintegrate, the conductors probably will!

Two actions of the OCPD’s are therefore important in protecting circuit wires & cables. These are: a) the speed of the clearing, and, b) how much ‘let-through’ current it allows to flow into the conductor. Therefore, when selecting an over-current protective device to protect a conductor, these questions must be answered:

a) Is the ampere rating of the OCPD matched with the “net ampacity” of the conductor? (This takes care of the overload condition).

b) Will the KAIC Rating of the OCPD be able to withstand the fault duty at the point of use? (This takes care of the possible disintegration of the circuit breaker).

c) Will the OCPD protect the conductors from disintegrating? (This takes care of the disintegration & vaporization of circuit conductors).

NEC 110.10 says that although conductors do have allowable ampacity ratings, they also have maximum allowable “short-circuit current withstand” ratings. Damage ranging from slight degradation of insulation to violent vaporization of the conductor metal can result if the short-circuit withstand is exceeded.

Scenario: Given a # 12 THW wire carrying a normal load of 10 amperes, and the short circuit current happens to be 35,000 amperes, if the operating speed of the circuit breaker is 1 cycle (1/60 or 0.0167 s), the amount of energy that the conductor will experience before the opening of the circuit shall be: 35,000 x 35,000 x 0.0167!

Do you think the # 12 wire can withstand 35 kA for a period of 0.0167 second? Maybe yes…, maybe not. What if not? In this case, one may consult available information such as the “Short-Circuit Characteristics of Cable” by ICEA (Insulated Cable Engineers Association, Inc.) & IEEE Color Books.

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(The above picture is a laboratory-simulated short-circuit to portray protection without due consideration to wire withstand limits. Note the melting of the insulation & the conductors and the subsequent flames.)

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Note that the mechanical over-current protective device (conventional circuit breaker) opening time effect should be known along with the available short-circuit current and cable withstand data to determine the proper conductor or OCPD that must be used. That’s why in cases of unusually high available fault current in the system, it is best to check sizes of conductors & corresponding type of OCPD’s to confirm whether the installation is safe from cable damage and where the overcurrent protective devices especially the slower mechanical OCPD’s are fit to be installed in the system.

As pointed out earlier, the key here is the speed of clearing by the breaker. If the breaker is slower than the 'short time withstand' of the wire, cable or bus; then the conductor disintegrates before the breaker completes its operation. And you need a much bigger conductor - much bigger than you can imagine for the usual ampacity requirements. If not practical both economically and physically, then means to limit the "let-through" current must be sought.

When we say ‘unusually high fault duty’, a good cue that warrants this review is a fault level of more than 25,000 amperes. It could mean that the source transformer may be too large for the application, say 1,000 KVA or 2,000 KVA for 240v and 480v respectively. In this case, a detailed fault calculation with all the system impedances must be accomplished.

As we now see it, short circuit can be more complicated than what meets the eyes. The truth is, the electrical engineer needs to upgrade his competency on the subject. Unfortunately, we can’t discuss this topic all in this blogsite. Its complexity, breadth and length need a more purposive seminar & training on the subject.


(To be continued… )

Next: “THE INTEGRITY OF ELECTRICAL INSTALLATIONS”