#### § 18.53 High-voltage longwall mining systems.

(a) In each high-voltage motor-starter enclosure, with the exception of a controller on a high-voltage shearer, the disconnect device compartment, control/communications compartment, and motor contactor compartment must be separated by barriers or partitions to prevent exposure of personnel to energized high-voltage conductors or parts. In each motor-starter enclosure on a high-voltage shearer, the high-voltage components must be separated from lower voltage components by barriers or partitions to prevent exposure of personnel to energized high-voltage conductors or parts. Barriers or partitions must be constructed of grounded metal or nonconductive insulating board.

(b) Each cover of a compartment in the high-voltage motor-starter enclosure containing high-voltage components must be equipped with at least two interlock switches arranged to automatically deenergize the high-voltage components within that compartment when the cover is removed.

(c) Circuit-interrupting devices must be designed and installed to prevent automatic reclosure.

(d) Transformers with high-voltage primary windings that supply control voltages must incorporate grounded electrostatic (Faraday) shielding between the primary and secondary windings. The shielding must be connected to equipment ground by a minimum No. 12 AWG grounding conductor. The secondary nominal voltage must not exceed 120 volts, line to line.

(e) Test circuits must be provided for checking the condition of ground-wire monitors and ground-fault protection without exposing personnel to energized circuits. Each ground-test circuit must inject a primary current of 50 percent or less of the current rating of the grounding resistor through the current transformer and cause each corresponding circuit-interrupting device to open.

(f) Each motor-starter enclosure, with the exception of a controller on a high-voltage shearer, must be equipped with a disconnect device installed to deenergize all high-voltage power conductors extending from the enclosure when the device is in the “open” position.

(1) When multiple disconnect devices located in the same enclosure are used to satisfy the above requirement they must be mechanically connected to provide simultaneous operation by one handle.

(2) The disconnect device must be rated for the maximum phase-to-phase voltage and the full-load current of the circuit in which it is located, and installed so that—

(i) Visual observation determines that the contacts are open without removing any cover;

(ii) The load-side power conductors are grounded when the device is in the “open” position;

(iii) The device can be locked in the “open” position;

(iv) When located in an explosion-proof enclosure, the device must be designed and installed to cause the current to be interrupted automatically prior to the opening of the contacts; and

(v) When located in a non-explosion-proof enclosure, the device must be designed and installed to cause the current to be interrupted automatically prior to the opening of the contacts, or the device must be capable of interrupting the full-load current of the circuit.

(g) Control circuits for the high-voltage motor starters must be interlocked with the disconnect device so that—

(1) The control circuit can be operated with an auxiliary switch in the “test” position only when the disconnect device is in the open and grounded position; and

(2) The control circuit can be operated with the auxiliary switch in the “normal” position only when the disconnect switch is in the closed position.

(h) A study to determine the minimum available fault current must be submitted to MSHA to ensure adequate protection for the length and conductor size of the longwall motor, shearer and trailing cables.

(i) Longwall motor and shearer cables with nominal voltages greater than 660 volts must be made of a shielded construction with a grounded metallic shield around each power conductor.

(j) High-voltage motor and shearer circuits must be provided with instantaneous ground-fault protection of not more than 0.125-amperes. Current transformers used for this protection must be of the single-window type and must be installed to encircle all three phase conductors.

(k) Safeguards against corona must be provided on all 4,160 voltage circuits in explosion-proof enclosures.

(l) The maximum pressure rise within an explosion-proof enclosure containing high-voltage switchgear must be limited to 0.83 times the design pressure.

(m) High-voltage electrical components located in high-voltage explosion-proof enclosures must not be coplanar with a single plane flame-arresting path.

(n) Rigid insulation between high-voltage terminals (Phase-to-Phase or Phase-to-Ground) must be designed with creepage distances in accordance with the following table:

Minimum Creepage Distances

| Phase to phase voltage | Points of measure | Minimum creepage distances (inches) for comparative tracking index (CTI) range1 | | | |
| --- | --- | --- | --- | --- | --- |
| CTI≥500 | 380≤CTI<500 | 175≤CTI<380 | CTI<175 |
| 2,400 | 0-0 0-G | 1.50 1.00 | 1.95 1.25 | 2.40 1.55 | 2.90 1.85 |
| 4,160 | 0-0 0-G | 2.40 1.50 | 3.15 1.95 | 3.90 2.40 | 4.65 2.90 |
| | | | | | |
| --- | --- | --- | --- | --- | --- |
| 1 Assumes that all insulation is rated for the applied voltage or higher. | | | | | |

(o) Explosion-proof motor-starter enclosures must be designed to establish the minimum free distance (MFD) between the wall or cover of the enclosure and uninsulated electrical conductors inside the enclosure in accordance with the following table:

High-Voltage Minimum Free Distances (MFD)

| Wall/cover thickness (in) | Steel MFD (in) | | | Aluminum MFD (in) | | |
| --- | --- | --- | --- | --- | --- | --- |
| A1 | B2 | C3 | A | B | C |
| 1/4 | 2.8 | 4.3 | 5.8 | 4 NA | 4 NA | 4 NA |
| 3/8 | 1.8 | 2.3 | 3.9 | 8.6 | 12.8 | 18.1 |
| 1/2 | \* 1.2 | 2.0 | 2.7 | 6.5 | 9.8 | 13.0 |
| 5/8 | \* 0.9 | 1.5 | 2.1 | 5.1 | 7.7 | 10.4 |
| 3/4 | \* 0.6 | \* 1.1 | 1.6 | 4.1 | 6.3 | 8.6 |
| 1 | (\*) | \* 0.6 | \* 1.0 | 2.9 | 4.5 | 6.2 |
| | | | | | | |
| --- | --- | --- | --- | --- | --- | --- |
| **Note:** \* The minimum electrical clearances must still be maintained. | | | | | | |
| 1 Column A specifies the MFD for enclosures that have available 3-phase bolted short-circuit currents of 10,000 amperes rms or less. | | | | | | |
| 2 Column B specifies the MFD for enclosures that have a maximum available 3-phase bolted short-circuit currents greater than 10,000 and less than or equal to 15,000 amperes rms. | | | | | | |
| 3 Column C specifies the MFD for enclosures that have a maximum available 3-phase bolted short-circuit currents greater than 15,000 and less than or equal to 20,000 amperes rms. | | | | | | |
| 4 Not Applicable—MSHA doesn't allow aluminum wall or covers to be 1/4 inch or less in thickness (Section 18.31). | | | | | | |

(1) For values not included in the table, the following formulas on which the table is based may be used to determine the minimum free distance.

(i) Steel Wall/Cover:

[![](https://img.federalregister.gov/ER23NO04.000/ER23NO04.000_large.png)](https://img.federalregister.gov/ER23NO04.000/ER23NO04.000_original_size.png)

(ii) Aluminum Wall/Cover:

[![](https://img.federalregister.gov/ER23NO04.001/ER23NO04.001_large.png)](https://img.federalregister.gov/ER23NO04.001/ER23NO04.001_original_size.png)

Where C is 1.4 for 2,400 volt systems or 3.0 for 4,160 volt systems, Isc is the 3-phase short circuit current in amperes of the system, t is the clearing time in seconds of the outby circuit-interrupting device and d is the thickness in inches of the metal wall/cover adjacent to an area of potential arcing.

(2) The minimum free distance must be increased by 1.5 inches for 4,160 volt systems and 0.7 inches for 2,400 volt systems when the adjacent wall area is the top of the enclosure. If a steel shield is mounted in conjunction with an aluminum wall or cover, the thickness of the steel shield is used to determine the minimum free distances.

(p) The following static pressure test must be performed on each prototype design of explosion-proof enclosures containing high-voltage switchgear prior to the explosion tests. The static pressure test must also be performed on every explosion-proof enclosure containing high-voltage switchgear, at the time of manufacture, unless the manufacturer uses an MSHA accepted quality assurance procedure covering inspection of the enclosure. Procedures must include a detailed check of parts against the drawings to determine that the parts and the drawings coincide and that the minimum requirements stated in part 18 have been followed with respect to materials, dimensions, configuration and workmanship.

(1) *Test procedure.*

(i) The enclosure must be internally pressurized to at least the design pressure, maintaining the pressure for a minimum of 10 seconds.

(ii) Following the pressure hold, the pressure must be removed and the pressurizing agent removed from the enclosure.

(2) *Acceptable performance.*

(i) The enclosure during pressurization must not exhibit—

(A) Leakage through welds or casting; or

(B) Rupture of any part that affects the explosion-proof integrity of the enclosure.

(ii) The enclosure following removal of the pressurizing agents must not exhibit—

(A) Visible cracks in welds;

(B) Permanent deformation exceeding 0.040 inches per linear foot; or

(C) Excessive clearances along flame-arresting paths following retightening of fastenings, as necessary.

[[67 FR 10999](https://www.federalregister.gov/citation/67-FR-10999), Mar. 11, 2002; [69 FR 68078](https://www.federalregister.gov/citation/69-FR-68078), Nov. 23, 2004; [69 FR 70752](https://www.federalregister.gov/citation/69-FR-70752), Dec. 7, 2004]