PTCB exam format

monitoring Index check | Killexams.com Resources

In an prior article, I lined fire Hazard exams that kind an important part of defense trying out for electronic/electrical products. We looked on the standards and gadget used for abnormal warmth, glowing wire and flame exams. a standard compliance check file for an equipment, such as a toaster, can be a reasonably long document reporting the consequences for a huge number of tests. amongst these, the area for “heat and fireplace” will always have the consequences of a 3rd verify – monitoring. It’s a phenomena most of us have followed, but needs some rationalization to remember what it skill.

monitoring is a surface phenomena on an insulating cloth. if you have two conducting terminals or tracks at a excessive voltage (larger than 100 VAC) separated via an insulator, a mixture of environmental components akin to dirt, moisture and thermal biking could trigger minute leakage currents to flow on the surface between the conductors. Over time, the deposits carbonize and the surface present increases. ultimately, a carbon tune forms over the surface of the insulator making it conductive at a selected “monitoring” voltage. finally, a brief circuit is created between the two conductors which may also also cause hearth. Worse, it’s feasible that the tracking present may be lessen than the rating of the defensive fuse within the equipment, so they can prevent the electrical provide from being cut off, developing a fire hazard. tracking may also be averted by using the right kind of insulating materials and adequate creepage and clearance distances. one of the crucial reasons for adding a slot between adjacent high voltage terminations or tracks on a PCB is to cope with monitoring.

It’s unimaginable to habits such exams in response to precise world conditions, so a standardized system is needed that can produce outcomes that enable distinct materials to be compared. The IEC’s Technical sub-committee 15E turned into in the past entrusted with the work of developing and preserving tracking index methods and requisites. considering that the magnitude of this usual and its broad implications, this work is now handled through TC 112 — comparison and qualification of electrical insulating substances and methods.

TC 112’s document IEC 60112 defines a “standardized method for the resolution of the proof and the comparative tracking indices of strong insulating materials” for voltages up to 600 VAC, and offers guidance on how to design a suitable test device. The ASTM has an equivalent doc — ASTM D3638 as does the UL — UL 746A-24. A extra extreme verify is covered below IEC 60587 — “Electrical insulating materials used below extreme ambient situations – test strategies for evaluating resistance to monitoring and erosion”. This test is frequently referred as the inclined airplane tracking and erosion check and specifies examine voltages up to six kV. but for now, let’s simply look on the low voltage check as per IEC 60112.

A sample of at least 20 mm x 20 mm with a minimum thickness of 3 mm is required for trying out, with a set of 5 samples being tested every time. If the examine product cannot give a pattern of those dimensions, then tiles of the insulating cloth deserve to be above all produced using the identical moulding technique as utilized in precise production. The sample is supported on a horizontal glass platform. Two chisel-edged platinum electrodes are positioned over the sample, separated by a gap of 4 mm. A voltage adjustable between one hundred to 600 VAC is utilized to these electrodes. The electrodes crush on the sample with a force of 1 N by way of dead weights.

The electrical deliver to the electrodes must be existing confined. For all voltages between a hundred V to 600 V, the brief circuit current across the electrodes need to be restrained to 1 A. this is always accomplished by means of capability of a series variable resistor (rheostat). In some equipment designs, the Variac (variable auto-transformer) for adjusting the voltage is routinely coupled to the rheostat ensuring the short circuit latest is always constrained to 1 A. An further, smaller cost rheostat is used for minor trimming. The average extra specifies that after atmosphere the open circuit voltage, the measured voltage at 1 A latest should now not drop by more than 10% (load law). This makes transformer design a little bit elaborate. At low voltages, there isn’t enough magnetic coupling between the windings, inflicting greater drops at lower voltages. One solution is to use two secondary windings of about 350 V each which are linked in parallel for test voltage below 300 V, and in collection for larger voltages. however there are alternative routes of fulfilling this requirement too. It’s just one example of how the clothier must examine every requirement in the typical and then figure out a way to enforce it in the check equipment.

The short-circuit latest is barely a limiting requirement of the electrical supply connected to the electrodes. The extra crucial atmosphere is the “tripping” existing which has to be set to 0.5 A above which the supply have to be disconnected from the electrodes. The tripping sensor needs to have a time delay of two seconds earlier than it journeys and the explanation for this surroundings will turn into clear a little later.

Environmental infection is simulated through a salt solution — always ammonium chloride having a attention of 0.1%. an alternate answer is prescribed for greater stringent trying out. while applying the verify voltage across the electrodes, one drop of the electrolyte is dropped over the check sample between the electrodes every 30 seconds for a complete of 50 drops. The measurement of each drop has to be adjusted such that fifty drops weigh roughly 1.075 grams and 20 drops weigh 0.430 grams. This will also be executed by using careful option of the needle diameter used for the drops as well as the beginning mechanism. Some designs use a gravity feed, solenoid operated gadget while others use a peristaltic pump. yet another method is to make use of an air pump which forces the liquid out of its container through forcing air in to it. The examine pattern passes if it survives 50 drops without triggering the over existing sensor. The sample fails if the over-present sensor receives prompted or if it catches hearth, at which aspect the electrical give has to be disconnected immediately.

When a drop falls over the pattern across the electrodes, many of the electrical present flows during the liquid for the reason that it's conductive. This reasons a existing spike that at once boils off many of the salt solution, and generally lasts for a 2d or two. during this two-2nd period, the over-existing device is programmed no longer to trip. With most of the water having evaporated, one of the most salt is left in the back of as a deposit over the pattern, which motives “monitoring” latest to circulate over its floor. a while later, you'll also observe some scintillation impact (sparking) as the leftover salt crystals burn out when the latest flows via them.

The outcomes of a monitoring check are stated in two other ways. A Proof tracking Index verify (PTI) is continually conducted at 175 V to verify that the pattern can live to tell the tale 50 drops. then again, a Comparative monitoring Index look at various is carried out over a range of voltages, incrementing the examine voltage by means of 25 V for each succeeding verify. The variety of drops is always set at 50. The CTI value is decided as the optimum voltage at which the sample withstands 50 drops. In some situations, the pattern should additionally pass the test at 25 V less than the CTI voltage for a period of one hundred drops. counting on the CTI value, the insulator is assigned a efficiency degree class with PLC0 being the highest and PLC5 being the lowest.

It’s all the time charming looking at a sample undergoing the tracking Index look at various — take a look at the video below. in case you study records sheets for plastic materials, the tracking Index value will all the time be suggested beneath it’s electrical residences. Paper Phenolic, which changed into the PCB substrate used earlier than the advent of fibreglass, constantly has a very low tracking index price (counting on its composition), ranging between a hundred V to one hundred seventy five V. having said that, reckoning on composition and filler substances, fibreglass substrates such as FR4 can have CTI values ranging from one hundred seventy five V up to about 300 V or better.

when you have ever seen a PCB (now not the accessories on it), provide off Magic Smoke, then you definitely’ve seen the effects of tracking in motion. With first rate design, taking into consideration proper creepage and clearance distances, it is one of the failure modes which may also be averted.