<ol>
	<li style="text-align:justify">
	<p><span style="font-size:14px">Vulnerability to Voltage Spikes: Such voltage transients can occur due to:</span></p>

	<ol>
		<li style="list-style-type:none">&nbsp;</li>
	</ol>
	</li>
</ol>

<p style="text-align:justify">&nbsp;</p>

<ol style="list-style-type:lower-roman">
	<li style="text-align:justify">
	<p><span style="font-size:14px">Switching operations of the breakers in primary circuit of C. T. and P. T.</span></p>
	</li>
	<li style="text-align:justify">
	<p><span style="font-size:14px">Lightning strokes.</span></p>
	</li>
	<li style="text-align:justify">
	<p><span style="font-size:14px">Breaking of control circuits.</span></p>
	</li>
</ol>

<p style="text-align:justify">&nbsp;</p>

<p style="text-align:justify"><span style="font-size:14px">Such voltage spikes can seriously damage the semiconductor components and in the worst case they may be rendered useless. But the advent of good filtering of the auxiliary circuits and surge diverters have made the static relays immune to such voltage spikes. The testing requirements are that the static relay must withstand, without being damaged, a power frequency a. c. high voltage of 2500 V between all terminals shorted on one side and an earthed terminal on the other side. Also, between same terminals impulse voltage of 5 kV, 1.2/50 microseconds is applied and the relay must withstand that.</span></p>

<p style="text-align:justify">&nbsp;</p>

<ol start="2">
	<li style="text-align:justify">
	<p><span style="font-size:14px">Variation of Characteristics with Temperature and Age: This was a demerit in early days when static relays were introduced. But now by appropriate use of thermistors, temperature compensating devices, better heat sinks and much better semiconductor technology, this is no more a problem.</span></p>
	</li>
	<li style="text-align:justify">
	<p><span style="font-size:14px">Overload Capacity: Once again, this was an issue in the days of introduction of static relays. In modern times, the continuous overload capacity of static relays is 400 % of relay rating.</span></p>
	</li>
	<li style="text-align:justify">
	<p><span style="font-size:14px">Reliability: Electromechanical relays used to have high reliability because they have smaller number of components. Whereas, as static relays are having large number of components to connect their circuitry, there is a doubt about their reliability. Careful choice of discrete components, a very strict quality control while manufacturing and accelerated life tests have considerably improved the reliability of the static relays. Figure 6.16 shows the reliability index of static and electromagnetic relays. Electromagnetic relays possess a reliability index of 100 % from the day one of their commissioning. But with passage of time, because of friction, bearing deterioration, deterioration in spring tensions, weakness in magnetic field strengths of permanent magnets etc. will reduce reliability index after 10 years or 15 years (in designs of renowned manufacturers). After about 20 years the reliability would have drastically reduced rendering these relays useless. </span></p>
	</li>
</ol>

<p style="margin-left:24px; text-align:justify">&nbsp;</p>

<p style="margin-left:24px; text-align:justify"><img alt="" src="https://edupmquestionhelp.s3.ap-south-1.amazonaws.com/hint/uploads/images/16251281591625128159.jpg" style="height:624px; width:800px" /></p>

<p style="margin-left:24px; text-align:justify">&nbsp;</p>

<p style="margin-left:24px; text-align:justify"><span style="font-size:14px">In static relays, on the other hand, the reliability index is very poor in early years of commissioning because reliability of many components cannot be guaranteed. But if these components are found proven up to 1 or 2 years, then for a very long time (may be 25-30 years or more), nothing will happen to these relays and will serve for many years. Now, if static relays are installed with rated current and voltage conditions for these 1-2 years in the manufacturing industry, reliability further is fully guaranteed. This is not economically viable. Hence accelerated life tests are conducted on relays at elevated current, voltage and temperature in disturbed atmospheric conditions created deliberately (such as high pressure than atmospheric, high humidity etc.) for few days or may be a month which is equivalent to 1-2 years.</span></p>

<p style="margin-left:24px; text-align:justify">&nbsp;</p>