• Electrical India
  • Dec 5, 2017

Common Errors While Designing an Electrical Protective Scheme

If the selection of inappropriate specifications switchgears go unnoticed, it may jeopardize safety of man and machines both. Therefore, utmost care is required, while designing an electrical protective scheme as far as selection of CTs of correct specifications, switchgears, etc is concerned…

- Ajay Singh


 In course of inspections of the respective electrical installations, numerous discrepencies pertaining to electrical schemes of switchgears are observed. Some of the discrepencies which are observed on regular basis, are elaborated hereunder:

• Use of metering CT in place of protective type
• Connection of both metering and protective apparatus through the same CT
• Use of switchgear of improper specifications in a particular protective scheme

The above-mentioned diiscrepencies are explained below:

Use of Metering CT in Place of Protective Type

  Shown in figure 1 is an electrical protective scheme known as combined over-current and residual earth fault scheme, which is usually employed in the electrical installations. As shown, the CTs continuously sense the current of circuit to be protected and feed the same to relays. The current so measured varies in magnitude and direction depending upon the type of fault.

Figure 1: Electrical protective scheme

  The B-H or I-V charecteristics of metering and protection CTs are shown in Figure 2 through a common curve using blue and green shades respectively. As can be seen, the core of metering CT gets saturated nearly at the rated current of CT whereas the same in case of protection CT happens at about 20 times of rated current ( typically for a 5P20 CT). Evidently, a saturated CT doesn’t pass on the prortionate current to the relay, resulting in its failure to function even in case of a severe short-circuit in the power line, endangering safety of human beings and electrical apparatus.

Figure 2: B-H or I-V charecteristics of metering and protection CTs

  A typical schematic layout is shown in figure 3 where metering CTs were being used inadvertently in place of protecticve ones and a short-circuit fault of say 2000A has occcurred in the outgoing side of circuit to be protected. The relays cease to function for want of current as against 50A of current during fault only 5A current flows through the respective relays, inadvertently making them defunct.

Figure 3: Schematic Layout

  It is worthwhile to advise herewith that the appropriate CTs shall be used, particularly, in the protective circuits so as to avoid the malfucntioning of relays in the event of exigency.

Connection of Both Metering & Protective Apparatus Through The Same CT

  In many electrical installations it is observed that both metering and protective devices are fed through the common CTs, making the scheme unrelaible, unsafe and ineffective in long run. An ammeter is designed to display the full load current and may carry a certain amount of over-current for a certain duration ( typically 2 to 2.5 times of rated current for about 10 sec or so). A typical scheme where both protective relays and metering devices are connected through the same set of CT is shown in figures 4 and 5.

Figure 4: A typical scheme where both protective relays and metering devices are connected through the same set of CT

  During short-circuit fault, very high curent flows which may lead to open-circuiting of ammeter. The open-circuiting may also take place due to failure of Ammeter Selector Switch, connected in the protective circuit as shown. The open-circuit in the protective circuit will lead to the dysfunction of concerned over-current relay and burning of concerned CT in the long run.

Figure 5: A typical scheme where both protective relays and metering devices are connected through the same set of CT

  In view of above, the metering and protective devices shall be supplied through the independent cores of a CT of appropriate specifications.

Use of Switchgear of Improper Specifications in a Particular Protective Scheme

  Adoption of wrong type of protective scheme is observed on regular basis in some of the electrical installations. An electrical network with the maximum short-circuit fault level of say 25 kA found being controlloed using a switchgear of breaking capacity of say 4.5kA, may lead to burning of switchgear itself in the process of clearing the fault. A typical scheme, being used to control the frequnet start/stop of machine, is equipped with instantaneous/high-set over-current protection. The maximum short circuit current for the existing impedance of installation is measured to have a magnitude of say 12kA and the relevant relay is wired to trip the concerned air-break/vacuum /SF6 type contactor having breaking capacity of say 4.5kA. Though the relay will actuate on sensing the fault but it is certain that the contactor alongwith relevant electrical panel may burn in the processs of clearing the fault.

Figure 6: A typical scheme, being used to control the frequnet start/stop of machine, is equipped with instantaneous/high-set over-current protection.

  In order to avoid incidents of above-mentioned type, it is mandatory to ascertain the suitability of relevant switchgears for a particular application.

Conclusion

  Through this article, an effort has been made to highlight some of the common mistakes committed while designing an electrical protective scheme which if adopted in the initial phase and gone unnoticed, may jeopardize the safety of man or machine or both.


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