Insulating Mats

While everyone was absorbed in the landslide technological developments taking place in communication and protection fields of Electrical Engineering, a silent revolution has taken place in rubber mats laid in front of switchboards as a means of providing safety for operators from electric shock. Gone are the days when mats looked like foam beds! The new mats are so thin, that you would wonder whether they would fly off if you switch on a man-cooler. Yet, these ultra thin mats are so effective that the industry has switched over to them. What follows below is a comparison between the old and new mats and a synopsis of their characteristics.

New Mats conforming to IS 15652:2006	Old Mats conforming to IS 5424:1969
Made of synthetic elastomer	Made of vulcanized rubber
Notified for use in India for all installations after Nov 1, 2007 through Gazette notification SO-2086	Superseded from Nov 1, 2007
High electrical insulation  resistance 10,00,000 mega ohm when measured with 500 V meggar	Comparatively far lower insulation resistance
100%  shock  proof  under  leakage  current 10 mA	Unsafe     in     case     of     leakage     of     current. Tested   from   small   electrodes   as   per   IS: 2584-1963 the value of the leakage current is 160 mA per sq. mtr.
Fire Retardant	High fire prone & fire encouraging
No effect of (a)  Transformer oil (b) Acid (c) Alkali (d)  Diesel	Affected by all these
The withstand voltage is 36kV  for one minute for 3mm mat	The withstand voltage is 15 kV for one minute
High dielectric strength 65KV for 3mm mat	Only 40 KV
Moisture proof	Absorbs moisture
Pastable type permanent fixed smooth trolley movement	Cannot be fixed permanently
Washable - Easy to clean	Not possible to clean
High tensile & elongation properties to withstand good mechanical properties.	No such properties as mat has to be removed for movement of trolleys

The voltage class and thickness of mats are as follows

New Mats conforming to IS 15652:2006		Old Mats conforming to IS 5424:1969
Thickness (mm)	Voltage class (kV)	Thickness (mm)	Voltage class (kV)
2.0	3.3	not less than 6.5 mm	not exceeding 3.3 kV
2.5	11
3.0	33
3.5	66

Spatial Clearances for Transformer Installation

Transformers are either installed indoors or outdoors. If two transformers are installed side-by-side, they shall be separated by fire-separation walls. Fire separation walls are deemed to be adequate even if oil capacity of individual transformers does not exceed 2000 litres, and total capacity of all transformers installed side by side exceeds 2000 litres^{Note 1}. Ventilation is the most important thing to be ensured in indoor installations. The area required for ventilation is also specified as 2 m² for outlet and 1 m² for inlet per 1000 kVA of transformer capacity^{Note 2}. This area is required for natural ventilation. If that much space is not physically available, fans may be used.

Vertical Clearance

Minimum clearance between the highest point of the conservator tank to the ceiling of the transformer room should be sufficient to remove the mounting on the transformer like the conservator. However, this clearance should not be less than 0.5 m ^{Note 3}.

Horizontal Clearance

Transformers shall be kept well away from walls. The minimum recommended spacing between the walls and transformer is shown in Fig 1 below.

Fig 1: Minimum horizontal clearances for transformers

 The wall indicated in the figure can also be fire separation wall. As such, there is no separate requirement for clearance from fire separation wall.

When fire safety is also taken into the picture, some more spatial clearances are to be observed. If transformer oil capacity is more than 2000 litres, the building housing the oil-filled transformer shall be separated by a distance of not less than 6 m from all other buildings ^{Note 4}. If however, a building is existing within 6 m, there shall not be any door or window opening in the substation or the adjacent building. Again, if such communication also exists, the substation shall be segregated by separating walls of 355 mm thick brick wall or 230 mm thick RCC, carried up to roof level. Doors, if any, shall be single fireproof with 2 h rating.

In the Code of Practice for Fire Safety: Electrical Installations (IS 1646), the minimum clearance specified is not in agreement with Code of Practice for Installation of Transformers (IS 10028 Part 2) when compared with Fig 1 reproduced from the latter standard. This specifies a minimum clearance of 750 mm (0.75 m) between the transformer or other apparatus and enclosing or separating walls ^{Note 5}. As is usual in the case of conflicts among provisions in standards, we may take the more stringent requirement of the two, which is, 1.25 m for transformers with enclosing walls on all four sides.

No mention of spatial clearances for transformers was found in the Indian Electricity Rules, 1956 and the Central Electricity Authority (Measures Relating to Safety and Electric Supply) Regulations, 2010.

American regulations seem to have included clear instructions on clearances as shown in this excellent article (Safety Clearance Recommendations for Transformer) on the Electrical Engineering Portal.

References

1. Code of Practice for Selection, Installation and Maintenance of Transformers, Part 2 Installation, IS 10028 (Part 2) – 1981, Reaffirmed 2001.
2. Code of Practice for Fire Safety of Buildings: Electrical Installations (Second Revision), IS 1646 – 1997, Reaffirmed 2002
3. Indian Electricity Rules, 1956
4. Central Electricity Authority (Measures Relating to Safety and Electric Supply) Regulations, 2010

Notes:

1. Clause 3.6.2 of IS 10028 Part 2 – 1981
2. Clause 7.3.1.1, ibid
3. Clause 7.3.1.5, ibid
4. Clause 7.1 of IS 1646 – 1997
5. Clause 7.4, ibid

Unused Taps of CT and Differential Relay

This is a case study of the unusual false tripping of a 110/11 kV, 15 MVA transformer in a large chemical plant. As a part of renovation, the old air-blast circuit breaker and current transformer were replaced as a retrofit. Brand new SF6 breaker and CT (both Crompton Greaves make) were installed on the 110 kV side. After commissioning, the load on the transformer was very low, normally about 1 MW as the transformer was feeding power to a section of the plant which was mothballed for a long time. When it was paralleled with two other similar transformers and loaded, it tripped on differential protection, causing production loss. Tests of insulation resistance showed no problem and after two spurious trips, it was decided to conduct a thorough study of the issue.

The details of the current transformer (CT) was 150/75/1 A, with three cores and three windings – The Class 0.5 winding for metering, Class 5P20 for protection and Class PS for differential relay. It was a multi-tap CT, with the terminal connections as shown below.

Nobody knew how the shorting link between 75A and 150A tap came about. It was assumed that since the CT came from the manufacturer with the terminals short-circuited, the commissioning staff might have let the shorting in place in order not to keep any portion of the CT winding in open circuit. It was suspected that this shorting link might be the cause of false tripping. A search on the Internet showed some examples where similar tripping of differential relay had occurred due to identical causes. But, nobody came out with a theoretical explanation or a practical illustration of the phenomenon. Those people simply declared that when they removed the shorting link, the differential relay did not trip thereafter.

So we decided to demonstrate what would happen with the shorting link in place. We conducted a primary current injection test on the CT terminals and measured the secondary current to the differential relay and in the shorting link. The readings are tabulated below.

Case A: With Shorting Link

Primary Injected Current (A)	Current to Differential Relay (A)	Current in Shorting Link (A)
30	0	0.37
60	0.02	0.65
75	0.2	0.78

 Case B: Without Shorting Link

Primary Injected Current (A)	Current to Differential Relay (A)
30	0.35
60	0.77
75	0.98

It is easily seen that practically no current flows to the differential relay when shorting link is present. The relay is set to trip at 30% of the normal current (1A), that is, 0.3 A. The second CT connected to the relay is on the transformer secondary (1000/1A) where no shorting link was present. So, when the secondary current reaches 300A, corresponding to a load of approximately 5 MVA, the differential relay, which is seeing only the secondary current will pick up and cause the transformer to trip. Only after the shorting link was removed did the CT supply the required current to the relay. The transformer was then successfully loaded.

Result: Don’t short-circuit the unused taps of CT winding if a differential relay is connected across one of the taps.