Interlocks and changer over requirements of DG / Transformer

Panel discussion on electrical safety regulations


Answer for Question 7 of the panel discussion on electrical safety regulations

dt 26 march 2022


Regulation 45 (interlocks) is under chapter 6 which is applicable for above 650 volt. Are they used for less than 650 volt as well?? How to understand the regulation.


Question 7. Regulation 45 (interlocks) under chapter VI:

Answered as two main questions

  1. Is it applicable for generator sets of voltage rating above 650V only? Is it applicable for generator sets of voltage less than 650 V as well? How to understand the regulation?

  2. Is four pole breaker a must as a control for such generator sets instead of TP breakers with neutral contactors for enabling interlock? (Question has been modified for clarifying a more specific nature of doubt)


Ans. 7 (i).

It is applicable for all generators irrespective of the voltage classification of the installation.


Explanations

1. Voltage classification of installation in the repealed IER 1956


The basic reason for such misinterpretations is due to the non-defining of voltage classification in the Regulations. In IER 1956, the classification is made under Rule 2 (av) as follows:


“Voltage” means the difference of electric potential measured in volts between any two conductors or between any part of either conductor and the earth as measured by a suitable voltmeter and is said to be;

“Low” where the voltage does not exceed 250 volts under normal conditions subject, however, to the percentage variation allowed by these rules;

“Medium” where the voltage does not exceed 650 volts under normal conditions subject, however, to the percentage variation allowed by these rules;

“High” where the voltage does not exceed 33,000 volts under normal conditions subject, however, to the percentage variation allowed by these rules;

“Extra high” where the voltage exceeds 33,000 volts under normal conditions subject, however, to the percentage variation allowed by these rules.


2. Present status on the classification of voltage of installation


The terms LV, MV, HV and EHV are legally incorrect since these are not specifically mentioned in the Regulations. In the specific definition under Regulation 2 (zzf), these voltage classifications are intentionally omitted despite the provisions of Act, 2003 under Section 2 (35), 53 and 73. Thus, the voltages alone are preserved throughout the Regulations instead of specifically classifying the type of installation as LV, MV, HV etc. The DISCOM, Inspectors and contractors are still using the words “LV, HV” etc. from the repealed IER 1956 instead of using the terms “electrical installations and apparatus of voltage not exceeding 650 volts” etc. from the new Regulations for easy understanding and for avoiding complex terms. The CEA must take a stand and notify the voltage classification on par with the IEC standards instead of describing an installation in a complex manner using phrases.


3. Voltage classification of installation Vs voltage rating of equipment


Coming to the point, it is important to do a job correctly. But it is more important to see whether a correct job is taken before doing it correctly. Hence it is more important to ascertain as to whether the 415 V generator sets are to be treated as covered by the provisions contained in the Regulations under the Chapter VI titled as “Safety provisions for electrical installations and apparatus of voltage exceeding 650 volts”.


The classification of voltages using phrases under various Chapters as exceeding 250V, not exceeding 650V etc. are to be treated as applicable for the electrical installation only and it is not for individual equipment. This intent can be understood from the relevant definitions of the Rule 2y of the repealed IER 1956 and CEA Regulation 2 (zb) both of which treat the word, specifically, for a composite electrical unit in a premises. This fact is strengthened from the various provisions, especially, Regulation 30 and 43. Hence it does not relate to a single equipment of any premises since any premises may contain equipment of various voltages. e.g. Regulation 45(2)(v) cannot be applied since it will be tough to find any generator with a voltage rating exceeding 650V will be of capacity say, 110 kVA. In other words, any generator set of rating exceeding 100kVA, 415V cannot be insisted to be protected for earth fault if it is placed in an installation with a voltage supply exceeding 650V. Similarly, it could be misinterpreted that earth leakage protective device stipulated under Regulation 42 need not be applied to an LV socket outlet in an industrial premises availing supply at a voltage exceeding 650V. The list will go endless and such an interpretation will result in chaos among the Inspectors.


It can also be noted that the BIS, IEC and other international standards in this matter are not referring the generators of voltage exceeding 650V. Applicability of 415V generators is clearly furnished in Clause 1.1.g of IS 732 as LV generators. Hence, it can be inferred that the recommendations of the Standards are based on the voltage rating of the equipment only and not based on the classification of installation voltage fed from the DISCOM.


The interlocks, neutral switching, earthing etc. covered in the Regulation 45 are squarely complementing the recommendations of IEC 60364, IS 732 and IS 3043. These recommendations are based on the voltage rating of the generators and not on the voltage classification of the consumer. e.g. It is practically seen that the same rating of say, 500kVA, 415V generators can be located in an MSB premises availing 415V supply from DISCOM or in an industrial or major residential complex premises with MSBs availing 11kV supply from DISCOM. In such cases, different norms cannot be insisted for the same voltage rating of the generators. In such a case, the entire IS or IEC recommendations referred here will not become acceptable


4. Misinterpretation


Of late Regulation 45 (1) is misinterpreted In Tamil Nadu, as applicable for installations exceeding 650V rating alone and hence the interlocking arrangement proposed for the 415V generators in multi storied buildings are rejected simply based on the voltage classification of “exceeding 650 V” under Chapter VI. Ironically, one cannot find any generator with a voltage rating exceeding 650V finding a place in a consumer premises availing 415V supply from DISCOM or even in some of the 11kV DISCOM supply and hence this Regulation cannot be applied to check the provisions of neutral switching among multiple generators in parallel operation and interlocking between the generator supply and the DISCOM supply.

The inspectors have not made such a misinterpretation throughout the period of IER, 1956 on the interlock arrangements in the case of parallel operation of generator sets since 1987. If the statutory authority goes on interpreting all such trivial things, literally, without a technical reasoning, then there won’t be any decision making and confusions will alone be rising.

5. Solution


Hence, I am of the view that Regulation 45 is applicable for all generators based on the voltage rating of the generators of voltage less than 650 V as well and it is irrespective of the voltage classification of the installation voltage fed by the DISCOM.


The CEA may kindly take a stand and notify the voltage classification on par with the IEC standards instead of describing an installation in a complex manner using phrases. In the meantime, a communication may be sent to the States for a correct interpretation, as the CEA consider fit, on this matter.


Q.7 (ii). Ans.


(ii)Is four pole breaker a must as a control for such generator sets instead of TP breakers with neutral contactors for enabling interlock?

No. Four pole breaker should not be insisted invariably in view of the restriction stipulated under Regulation 15. The fourth pole (neutral) breaking should be restricted to the changeover panel between DISCOM supply and generator supply only. Four pole breakers need not be insisted for the single generator control panel or synchronizing panel unless it becomes a common panel for change over arrangement also.


Explanations


1. Requirement for generator sources in an electrical installation

In the electrical installation, generators are operated for supply continuity in parallel operation mode among multiple generator sources as well as in an independent mode as an alternative source for the DISCOM source during power failure.

The wordings “multiple source”, “alternative source” is hence coined in IEC 60364 Part 4-44 as well as IS 732. In IS 3043, the wordings are used explicitly as “generator operating in isolation from other supplies”, “standby generators without mains paralleling facility” for describing the method of connections.

The following main requirements are considered essential for the operation of generator sources.


  • avoidance of mixing of neutral of generator and DISCOM source of supply using interlock/change over switch,

  • integrity of neutral,

  • proper interlocking logic between the neutral earthing circuit and the corresponding breaker of different generators in parallel operation,

  • single point earthing to avoid stray neutral current.


2. Standards and Regulation for reference

In respect of the installations involving different sources of supplies, norms to be adopted on the important factors like earthing and transfer of supply using change over arrangement are already in place under regulations and standards. Some of the national and international level standards and the corresponding regulations of this country supplementing each other are:


3. Prescribed Regulations and Standards

In TN systems the transfer from one supply to an alternative supply shall be by means of a switching device, which switches the line conductors and the neutral.

The relevant provisions of the Regulation and Standards are:


A) CEA Regulation 15(ii)- no cut-out, link or switch other than a linked-switch arranged to operate simultaneously on the earthed or earthed neutral conductor and live conductors shall be inserted or remain inserted in any earthed or earthed neutral conductor of a two wire-system or in any earthed or earthed neutral conductor of a multi-wire system or in any conductor connected thereto


Provided that the above requirement shall not apply in case of-

(a) a link for testing purposes, or

(b) a switch for use in controlling a generator or transformer


B) Regulation 45 (1) (vi). reads as follows:

where two or more generators operate in parallel and neutral switching is adopted, inter-lock shall be provided to ensure that generator breaker cannot be closed unless one of the neutrals is connected to the earthing system.


C) CEA Regulation 45 (iii) - where two or more supplies are not intended to be operated in parallel, the respective circuit breakers or linked switches controlling the supplies shall be inter-locked to prevent possibility of any inadvertent paralleling or feedback;


D) Cl. 551.6.2 of IEC 60364 Part 5-55 Standards reads as follows:


NOTE 1It may be desirable in TN systems to disconnect the neutral of the installation from the neutral or PEN of the system for distribution of electricity to the public to avoid disturbances such as induced voltage surges caused by lightning.”


4. Misinterpretation


I am reporting a typical case for this subject to show as to how the interpretations made by the Electrical Inspectors affects the observance of the statutory provisions:

The schematic arrangement showing the TP ACBs with neutral contactors and interlocking arrangement for the generator sets under parallel operation for an MSB installation is reproduced below:



Fig-1:SLD of MSB installation showing the Main LT change over panel and DG synchronizing panel



Fig-2:SLD of MSB installation showing the Main LT change over panel alone for clarity


Fig-3:SLD of MSB installation showing the synchronizing panel alone for clarity


The objection raised by the Inspectorate on the above schematic arrangement is reproduced below:




5. Difficulties faced in the Compliance of Regulations and Standards due to Misinterpretation


A. How does the four-pole breaker at the source end, affect the compliance of other provisions?

It should be ensured that compliance of 3C and 3D above does not result into a violation of 3A and 3B above. Provisions of 3C and 3D are explained in the IS 732 which follows IEC 60364 series, as per sketch furnished below:



NOTE: This method prevents electromagnetic fields due to stray currents in the main supply system of an installation. The sum of the currents within one cable must be zero. It ensures that the neutral current flows only in the neutral conductor of the circuit, which is switched on. The 3rd harmonic (150 Hz) current of the line conductors will be added with the same phase angle to the neutral conductor current.

Fig.4: IS 732-Figure 39 - Three-phase alternative power supply with a 4-pole switch


The interlock arrangement with details of breakers at the generator and load side are clearly shown in IS 3043 in the following sketch:


Fig 5: BIS 3043-FIG.39:LV Standby generators paralleled and with Neutral connected


Fig 6: Showing no break in neutral at source end


Fig.5 and 6 above shows fourth pole isolation at the load side only and not at the source side of the generators. Hence, it can be inferred from various standards and Regulation that four-pole break is permissible at the load end only and not at the generator end in respect of multiple and individual generator source and there is no conflict between the standards and Regulation.


It can be seen from the SLDs shown in Fig 1 to 3 above that four pole breakers are shown as incomers for the DG and DISCOM supply sources at the main change over panel only, to serve the purpose of changeover arrangement to comply 3C and 3D above. Hence, insisting such a four-pole breaker near the generators also, obstructs the required interlock on neutral switching for parallel operation as permissible by the Regulation 45 (1)(vi).


B. How does the single point neutral earthing affect the compliance of other provisions?

Another point is regarding the observance of single point earthing to avoid EMC issues as prescribed for multiple source power supplies as per cl. 4.5.4.4.6.1 and cl. 4.5.4.4.6.2 of IS 732. Such an arrangement is shown in the sketch furnished below:


a) No direct connection from either transformer neutral points or generator star points to earth is permitted.

b) The conductor interconnecting either the neutral points of transformers, or the star-points of generators, shall be insulated. This conductor functions as a PEN conductor and it may be marked as such; however, it shall not be connected to current-using-equipment and a warning notice to that effect shall be attached to it or placed adjacent to it.

c) Only one connection between the interconnected neutral points of the sources and the PE shall be provided. This connection shall be located inside the main switchgear assembly.

d) Additional earthing of the PE in the installation may be provided.


Fig 7: Figure 37 of IS 732– TN multiple source power supplies to an installation with connection to earth of the star points at one and the same point


This is the arrangement specific to the IT (information technology / offices with computers) installations for EMC mitigation and it is not specific to the interlocks etc. Also, this will not be sufficient to mitigate the EMC, noise voltage at low and high frequencies, since the downstream isolation transformers, UPS combinations requires special attention and connections governed by other Standards like IEC TS 61200-203 and IEEE 1100. Further, insulated cable for neutral and single point earth are impractical for distantly located generator sources in the Indian conditions. Any missing in such stringent measures could defeat the very purpose of single point earthing.


The practices and specific mitigation adopted in the IT installations in accordance with the Regulation and Standards in this country are proven and sufficient to achieve an analogous single point neutral earthing without affecting the integrity of the neutral.


Cl. 29.2.3.1.c. of IS 3043 prescribes the switching protocol at the neutral earthing circuit to achieve single point neutral earthing without affecting the integrity of neutral of the synchronized source under different conditions. As per this clause, neutral switching, independently from the breaker is required for providing interlocking between the neutral of different generator sources and the breakers of corresponding generators in such a way that the neutral of any one of the generators (during parallel operation) is in circuit and the neutral of functioning generator (when the parallel operation ceases to bring a single generator for supplying the load) is brought into circuit to ensure the integrity of neutral.


With the above logic, arrangement of single point neutral earthing obstructs the compliance of 3 B above.


6. Conclusion


Hence, In my opinion


(i) four-pole breaker is applicable for the load side only. Any additional provision at the source side of the generator except the case of a generator supply directly feeding the load will obstruct the compliance of interlocking arrangement permissible under Regulation 15(ii) and 45(1)(vi);


single point neutral earthing obstructs the compliance of interlocking arrangement permissible under Regulation 45(1)(vi); (or this regulation may be suitably modified to include single point neutral earthing??? E.g. where two or more generators operate in parallel, where single point neutral earthing is not carried out and neutral switching is adopted, inter-lock shall be provided to ensure that generator breaker cannot be closed unless one of the neutrals is connected to the earthing system)


Appavoo Subbaiya

Former CEIG/TN

appavoo_s@yahoo.com


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