Testing with my MOM!
Nils Wäcklén
Product Manager
 Low resistance ohmmeters are amazingly useful instruments, with applications that range from checking the condition of the resistance of circuit breaker contacts to confirming the integrity of welds in racing car safety cages. If they are to give reliable and meaningful results, however, and in some cases to ensure compliance with applicable standards, these instruments have to carry out tests at high currents – typically 100 A or more.
Therein lies a problem. A low resistance ohmmeter capable of delivering this much current is unavoidably heavy, bulky and expensive. That’s not really ideal when tests have to be carried out in locations that are hard to access, such as underground substations, deep within the bowels of a racing car, aero-plane maintenance or working at height. And there’s yet another issue – these instruments are necessarily mains powered, and there are many sites where arranging for a suitable supply is by no means easy.
But what’s the alternative? Until recently there wasn’t one, but in the last few months a new breed of digital low resistance ohmmeters has appeared, and these new instruments seemingly do the impossible.
They’re small, handheld units that weigh only around 1 kg – that’s around one-twentieth of the weight of a conventional high current low resistance ohmmeter – and they are, compared with their conventional counterparts, almost unbelievably inexpensive. In spite of their diminutive size, weight and price, these ground-breaking instruments can measure resistance at currents in excess of 200 A.
The benefits are easy to appreciate – we’re essentially talking about go-anywhere low resistance testers that are as convenient to handle and use as a multimeter. But how is it possible to get so much current out of such a small package? The answer is to use an ultra capacitor. In almost every way, these novel components are much the same as ordinary capacitors and just about the same size, but there’s one big difference. They have capacitance values measured in hundreds of farads rather than microfarads.
In outline, this is how one of the new generation low resistance ohmmeters works: first it charges the ultra capacitor then, to carry out the test, it discharges it through the test object while measuring current and voltage. The instrument uses these values to calculate the resistance of the test object. Since the ultra capacitor has a very high capacitance value, it can store enough energy to deliver a hundred amps or more for an appreciable length of time – certainly long enough for accurate and dependable results to be obtained.
Of course, there’s a little bit more than this involved in producing a practical instrument, but the principle is clear. In addition, it’s not difficult to see that an instrument of this type can be designed to work from batteries, which would charge the ultra capacitor thereby eliminating the need for a mains supply.
To provide an idea of what can be expected from these new handheld low resistance ohmmeters, the patent pending Megger MOM2 can deliver an initial test current of up to 220 A, and a guaranteed minimum current of 100 A for three seconds for each measure, which easily meets the requirements of both the IEC and IEEE standards for measuring contact resistance in high- and medium-voltage circuit breakers.
It has a measuring range of 1 µΩ to 1 Ω, with a resolution down to 1 µΩ, depending on the range, and it features rechargeable batteries with sufficient capacity to handle a full day of testing.
It is only to be expected that, in spite of the convenience of these new instruments, some users will have reservations about using them because they are used to working with traditional instruments that can deliver current continuously. In most cases however, this is not a real concern.
We have already noted that the new instruments fully meet international standards for checking circuit breaker contacts and, in other applications, like measuring the resistance of busbar joints or welds, the duration of the current flow is unimportant, provided that it is long enough to allow accurate results to be acquired.
In some cases a slight modification of working practice might be needed. For example, in the past it might have been the usual practice to inject a continuous current into a whole section of busbar and then measure the voltage drop across each joint with either the ohmmeter test leads or a voltmeter. Given the very portable nature of the new instruments, however, and their ease of use, an equally valid and convenient approach is to measure the resistance of each joint individually.
Only in a very few cases is the use of continuous test current essential, such as when it is necessary for the object under test to be heated by the passage of current over a period of time. In these cases, the new handheld low resistance ohmmeters are clearly not the right tool for the job, but it is worth reiterating that these situations are few and far between.
Lugging heavy test equipment around is never a pleasure and there are many instances where the use of bulky mains powered equipment is difficult if not impossible. The new handheld low-resistance ohmmeters provide a genuine and genuinely convenient alternative not only in these instances but in almost every situation where low resistance needs to be measured accurately with high test currents. |
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Love them or hate them!
People who would describe electricity pylons as beautiful are without doubt, in the minority. But that may be about to change as the Royal Institute of British Architects (RIBA) has recently launched an international pylon design competition with the objective of producing pylons with enhanced visual appeal.
For regular readers of Electrical Tester, this won’t come as too much of surprise, since in a recent issue we covered the efforts of installation artist Elena Paroucheva, who has transformed the appearance of four high-voltage transmission towers on the 225 kV line between the French cities of Amnéville and Montois. And she’s by no means alone in seeing pylons as potential artworks.
In 2010, the Boston Society of Architects Unbuilt Architecture Award was won by a design for humanoid pylons developed by Massachusetts-based architects Choi + Shine as an entry for a competition run by the Landsnet, the company that operates the electrical transmission system in Iceland. While pylons using these designs have not yet been built, it has been reported that Norway’s TSO Statnett is considering adopting some of them.
Do we really want or need to start dressing up on our pylons, or to contort them so that they resemble some kind of simulated human being?
Whatever happened to the admirable principle of “form follows function”?
Did Frank Lloyd Wright and the other functionalists live and die in vain? Let us hope not. That’s not to say that there is anything wrong in principle with revisiting the design of electricity pylons. The design currently in use in most of the UK is a product of the late 1920s, and it’s perfectly possible that there is now scope for improvement, especially given the availability of new materials and manufacturing techniques.
However, there’s no doubt that the existing UK design for pylons is a very hard act to follow. The design was developed at least in part, to appease the strong opposition in the late 1920s and 1930s to what many thought would be the despoiling of the beautiful English countryside by the intrusion of ugly industrial structures.
There was never any chance that the march of the pylons could be stopped – the only way to satisfy the fast-growing demand for a reliable supply of electric power was to link together the separate generating undertakings that had grown up in the early days of the power industry. And linking them invisibly with underground cables was simply not a viable option – underground cables typically cost between ten and twenty times as much as their aerial counterparts.
So the pylons were inevitable, especially as the growing political instability in continental Europe meant that the UK had to take every feasible step to ensure that the supply system would be capable of addressing the exigencies of war. The solution was a true English compromise – yes the pylons would go in, but the design adopted was far more elegant than that which was being used in other parts of the world.
The end result is that, in the UK, today’s pylons are still slim graceful structures that are superbly functional and, many would say, enhance rather than despoil the landscape. So come, budding architects and artists, enter the RIBA competition by all means – after all, even the best can always be improved.
But let’s eschew functionless prettiness that simply increases pressure on already tight-stretched budgets, and leads to structures the appeal of which is questionable even when they are being built, and turns to abhorrence in just a few short years. Nice 1960s tower block, anyone? And just remember how many awards they won in their time!
The primary purpose of electricity pylons is to facilitate the transmission of electrical power. That is; they are functional. In developing successors for the current classic design, let us hope, therefore, that the design gurus will strive for another classic design that that will stand the test of time and that they will resist the treacherous seduction of prettiness, remembering always that form should follow function. Then, perhaps, there will be rather more of us who find beauty in pylons.
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To pot or not - part 2
Jeff Jowett
Applications Engineer
In the previous issue of Electrical Tester we focused on insulation testing, in this second part of the article we’ll concentrate on high-pot testing.
High-pots differ in many ways from the insulation testers discussed in the first part of this article. Yes, they apply high voltage across insulation and measure leakage current, but the similarity ends there. If that wasn’t confusing enough, high-pots come in two completely different types. These are industrial high-pots and cable high-pots. Industrials are used (generally as a requirement) by manufacturers of electrical products to assure the safety of the product before it ships and falls into the hands of the user. Cable high-pots are employed by the utilities as a pre-conditioner in the fault-locating process, and in installation and maintenance checks of other high-voltage equipment.
The manufacturers of electrical goods are often required by various agencies to assure the public of the safety of their product by testing it to independent standards Organisations like UL, CSA and VDE (Underwriters Laboratories, Canadian Standards Association and Verband der Electrotechnik) require a final high-pot test for many types of product from consumer goods to commercial equipment. Hi-pot testers are radically different from insulation testers: they are generally ac and they do not provide a measurement. They are concerned only with pass/fail, and tolerate virtually no operator involvement.
High-pots that use ac are preferred because they simulate the voltage stress that will be experienced by most equipment when in operation. Because of the reversing polarity, ac tests tend to pick up end defects in windings better than dc, which concentrates stress more to the middle. However, many products possess considerable capacitance in their design (large windings, parallel runs of wiring and cable) so that charging currents “trip” the testers. With a dc insulation test, the trained operator knows to watch for the stabilization that comes with full charge. But with an ac test that never happens, unless the test item can be completely charged on a half cycle (before current flows the other way). This requires considerable power, large transformers, and prohibitively bulky, expensive and unwieldy test equipment. Accordingly, standards agencies generally allow a dc test to be substituted, with test voltage adjusted from RMS to ac peak.
Also called a “dielectric strength test”, a high-pot test is just that. It is concerned with the strength of insulation, its ability to withstand voltage stress, rather than with its measurement. Some high-pots do indeed include meters that indicate current or resistance, but these are ancillary functions.
The purpose of a hi-pot tester is to break down marginal insulation …literally kill the weak. Products manufactured with electrical defects - faulty components, incorrect assembly, loose strand - can present a serious, sometimes lethal hazard. Manufacturers don’t want them to get off the end of the assembly line. The high-pot stresses the insulation at high voltage. The most prevalent standard is twice rated plus one thousand volts, so that 120 V-rated items would be tested at 1240 volts. It is expected that the test will break down and short out any marginal items.
The final part of this article will provide further information about industrial hi-pot testers. It will also explain how cable hi-pots differ, as well discussing some of their applications. |
Follow the logical approach |
Allen Joyce
Product Manager
The key to the safe, rapid and efficient location of cable faults is to follow a logical approach. There is an order of doing things that should be adopted. Deviation from a logical approach gives rise to the possibility of coming to the wrong location or destroying a fault condition that would have helped in finding the fault location.
Safety
Before examining the logical approach to cable fault location in more detail, lets consider some important safety-related points, after all, electricity can be lethal and few people get more than one chance. These basics should always be considered before commencing cable fault location:
 These are absolute minimum safety requirements. In addition, at all times, the safety rules of the organisation for which the testing work is being performed must also be observed.
A conductor should never be touched until it has been earthed, and no conductor should ever be earthed until is has been tested and proved to be dead. The tester must itself be tested before and after the proving test.
All cables and equipment should be discharged after testing, even if the test equipment used incorporates automatic discharge.
The Logical Approach
Step 1 – Fault Identification
Typically, the faulted circuit will have been identified by the operation of protection devices and by reports from the field Engineer. The faulty cable should also have been isolated and made safe for testing. Whilst we know the faulted cable we do not know the faulted phase or phases.
Check the cable with a time domain reflectometer (TDR).
This will act as a continuity test and help the operator identify which of the core or cores of the faulted cable are faulty. Dependent on the impedance of the fault the resultant trace may actually also give the operator an “idea” of the fault location.
Additionally, ask about everything connected with the situation – for example, the type of cable, length, time in service, how the fault occurred, what protection operated and any tests already carried out. Get as much information as you can.
Believe only the facts. Never ever believe any opinions about the diagnosis or the possible fault location.
Check everything yourself.
Step 2 – Diagnosis
This is a crucial step because, if the diagnosis is wrong, it will probably prove impossible to locate the fault, as you could very easily have selected the wrong fault location method. If success is achieved despite a wrong diagnosis this is pure luck, which is definitely not something to depend on! A much better option is to proceed logically, as follows:
Test the insulation resistance between all conductors and the cable sheath, and record the values for all combinations. From this diagnosis we decide on which method of prelocation we use, dependent on whether it is a low resistance or high resistance fault. This insulation test should be undertaken with the minimum voltage necessary, so as not to change the fault condition.
Undertake a high voltage proof test to determine the amount of leakage current present and also the actual “break-down voltage” of the fault. This will assist you in deciding on which level of voltage should be applied during your high voltage prelocation. This procedure will achieve two things – it will assist with determining the type of fault, and it will make it easier to decide on the method of pre-location that’s likely to be most effective. It will also provide an invaluable record for future reference.
Step 3 – Pre-location
Pre-location is used to provide an indication of the distance to the fault. While occasionally it may be necessary to modify the fault in order to create conditions more suitable for a particular pre-location technique, it is always best, if possible, to pre-locate the fault with conditions as found. There are several recognised methods of pre-location that assist rapid, accurate and safe location of faults. These include:
Pulse Echo (low voltage pre-location)
Arc Reflection (high voltage pre-location)
Arc Reflection Plus (ARP)
Differential Arc Reflection (DART)
Impulse Current
Voltage Decay
The results obtained with these techniques will allow the approximate location of the fault to be determined, but the accuracy of the results is affected by many factors, including changes in cable types, cable size, joints which affect the velocity factor of the cable under test. The lay of the cable is an important factor as any results obtained with prelocation are relative to the actual length of the physical cable, which may be different to the indicate length of the cable route!
Step 4 – Pin-pointing
Pin-pointing is positive identification of the exact location of the fault. Pin-pointing is carried out directly over the cable. The most common technique relies on detecting acoustic and electromagnetic signals emmitted at the fault location when the cable is being surged by a surge generator (thumper). The signals are detected with a sensitive ground microphone and electromagnetic pickup, used in conjunction with an amplifier.
Step 5 – Confirmation
After the exact fault location has been confirmed by pin pointing, the cable must be excavated so that the fault can be confirmed visually. In some cases, the fault is obvious because of external signs such as cracks, breaks, burning and general damage. Often however, there may be no visible damage, the fault being contained inside an apparently sound cable.
Step 6 – Restoration
After the fault has been repaired, the cable must be re-tested to ensure that there are no other faults. Multiple faults are rare, but by no means unknown. Finally, it is good practice to record details of the fault, whether this record will be used only for future reference or to illustrate bad cable laying practice, poor jointing technique or to provide evidence for claims against third parties who may have created the fault by damaging the cable. |
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