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Yes! I do have! But how to send it to you? It is too large to be posted/pasted here. If you can infirm me your mail ID, then I can post you a few papers on the subject.
I am from India. We generallu use only Aluminium Cables, particulary at 0.4kV. We use 3 core or 3.5 Core, Aluminium conductor, PVC inner-sheathed, GI wire/strip armoured, PVC outer sheathed cables for 0.4kV system, if the cable is ti be buiried underground. For the installation procedure, we do follow our nation’s Standard Code of Practice for Cable installation. In India, we are governed by the Indian Standard Specification Number 1255, which is the “code of practice for selection, installation & maintenance of power cables upto 33, 000V”.
Perhaps due to the capacitance in the spike buster’s electronic circuitry. How long does the LEDs glow?
Fuses are vunerable at low fault levels (they take more time to operate than CBs)
Circuit breakers are vulnerable at high fault levels (They take more time to operate than fuses)
I would say that is an added precaution to earth the rear covers of panels too. Generally we do not do this as rear covers are screwed and as such while we access the rear covers, the panel would be made dead and hence no danger. Whereas front cover access can be when the panle is live too. Moreover, many a times the front panel would have on them live components such a spush buttos, indicationg lamps, selector switches, meters, etc. thus waranting a dedicated earth connection. As the rear doors would not have any such arrangemnet, we do not generally earth the rear doors with a dedicated earth connection.
Yes! Your interviewer was very correct! There is a very precise formula for sizing the earth strip:
S = I Root 't' / K
where, S is the cross sectional area of the earth strip, in sq.mm
I is the maximum anticipated earth fault current, in Amperes
t is the duration of the earth fault in seconds
K is a constant dependinng upon the material of the earth strip, type of insultaion & method of laying the earth strip (Pl. refer to IS 3043 for the values of these constants)
First of all, BIL is Basic Insulation Level and NOT basic impulse level, as mentioned by you incorrectly. The BIL includes the power frequency overvoltage withstand capacity and the impulse (both Lightning Impulse and
p the Switching Impulse) withstand capacity.Sub-station clearances are decided on the BIL, as any insulation or dielectric is stressed by these voltages.
Shawn said:
Anil
In the UK the quality of the electrical supply system is controled by the National Grid. The National Grid company has a set of standards that it refers to as the grid code. The code is the minimum standard of power quality that a generator supplies to the grid. If the generation does not meet the code it does not go onto the grid.
The code also sets standards for customers equipment if the running of the equipment can be reflected back to the grid. You would not want a factory with a large direct online motor depressing the local supply voltage everytime it is started.
In India too we do have Grid Codes but there are far too many short cuts to avoid penalty for violation.
Yes! I too have had similar experiences with more than one panel builder is India. Many a times panel builders have objected to installing space saving devices such as MPCBs. I was told by one panel builder that normally panel builders have huge margins in Sheet Steel, Bus bars & in wiring. But, with devices like MPCBs, Combination Starters, etc. there is no sheet steel, no bus bar (as these come with reday made bus bars) & no wiring. So, they object to these devices. I am not sre how far this is true. But, this came from a panel builder himself.
Unfortunately, there is no short cut formulae for calculating the current rating of cables. May be you will be able to calculate the DC Current rating of cables with respect to the cross sectional area fairly linearly, as the DC resistance would vary linearly with the cross section. But, in AC the resistance is affected by SKin & Proximity Effect and also reactances play a rule. Reactances do not vary in linear proportion with the cross section. So, best is to follow the manufacturer's rating tables.
