Features of power supply of devices in explosive zones.
Secondary power supply sources (SPS) redundant, intended to provide power to fire alarm systems located in explosive zones, must structurally comply with the requirements of Federal Law No. 123-FZ «Technical Regulations on Fire Safety Requirements», GOST R 53325-2009 «Fire-fighting equipment. Technical means of fire automation. General technical requirements. Test methods», as well as national explosion safety standards.
The use of secondary power sources is regulated by the Code of Practice SP 5.13130.2009 «Fire protection systems. Automatic fire alarm and fire extinguishing installations. Norms and rules for design», Electrical Installation Rules (PUE), section 7 «Electrical equipment for special installations».
According to the requirements for sources of the 1st category of power supply reliability, redundant secondary power supply sources provide power supply to fire alarm systems from two sources: from the power supply network via a step-down power source or from a storage battery that reserves power supply to the load when the power supply network is disconnected (clause 5.2.1.1 of GOST R 53325-2009). Therefore, redundant ES must contain a channel for powering the load from the network, a channel for charging the battery from the network, and a channel for discharging the battery into the load when there is no power supply network.
An important property of redundant IE is to ensure the functioning of automatic fire alarm systems (AFAS) when the main power supply is disconnected, and the characteristics of the battery affect the fulfillment of the conditions of clause 15.3 of SP 5.1313.2009 “… must provide power to the specified electrical receivers in standby mode for 24 hours plus 1 hour of operation of the fire automatic system in alarm mode.”
First of all, the «lifetime» is related to the load consumption currents, then the time depends on the battery capacity. There are different schemes for ensuring battery discharge, which differ in losses in the scheme and, ultimately, the possible depth of discharge. The discharge time can be roughly estimated based on the capacity of the battery used: 10-hour battery discharge at a current of 10% of the capacity of a fully charged battery, this ratio allows proportionally recalculating the «lifetime» of the ASPS for load currents when powered by a battery. To avoid damage to the battery, it is necessary to ensure that the battery discharge is limited to no less than the permissible value (determined by the TD manufacturer), and the charge level and rate are limited.
A mandatory condition for the source according to clause 5.2.1.9 of GOST R 53325-2009 is “The ES must maintain its parameters when the voltage at any input of the power supply changes from 85% to 110% of the nominal value”. Under these conditions, the network channel of the power source must fully operate, and the battery must be charged. If the ES ensures the operation of the network channel in an increased voltage range relative to the required one, this indirectly affects the “lifetime” of the ASPS, since the reason for switching to battery power is not only a complete shutdown of the power supply network, but also the output of the network voltage to levels that do not allow the load to be supplied via the network channel. Figure 1.
A pulse power supply, as a rule, has a wider range of input voltages than a transformer one. As a result, in case of a voltage drop, the network channel of the pulse power supply can continue to operate without using the battery to power the load.
To provide low-voltage power supply to the associated equipment in an explosive zone (in particular, ASPS devices and security systems), two approaches can be used:
1. The redundant ES is installed outside the hazardous area. In this case, the ES may not have explosion protection, but in the hazardous area, the power supply cable lines must have appropriate protection. The disadvantage of this power supply method is that it is necessary to take into account the inevitable large voltage drop with long power supply lines. In mines, tunnels and other industrial explosive facilities, the distances to the explosion-proof zone can be very large. This method provides centralized power supply to devices located in the explosive zone. Figure 2.
2. With the second method of power supply, the redundant ES is installed directly in the explosion-hazardous area. Then it is possible to build a distributed system for providing peripheral devices of the ASPS (and not only) with low-voltage power supply. In this case, the redundant ES can be installed in close proximity to energy consumers (light and sound alarms (boards), security and fire alarms with a 4-wire connection circuit, video cameras, low-voltage lighting devices, etc.), thereby avoiding large voltage drops. However, in this case, the redundant ES must have an explosion-proof design, which imposes additional requirements on its design. Figure 3.
The explosion-proof redundant IE must comply with explosion protection standards.
The enclosure of the IE must be made according to GOST R 51330.0-99 «Explosion-proof electrical equipment. General requirements», which immediately limits the possible design of the device. Quite often, IE enclosures are made of plastic materials (ABS plastic, polystyrene). For explosion-proof IE, the use of plastics for the enclosure is limited by clause 7.3.2 of GOST R 51330.0-99 — «Enclosures must be designed in such a way that under normal conditions of operation, maintenance and cleaning, the risk of ignition from electrostatic charges is excluded», this is, as a rule, a special antistatic plastic with an insulation resistance of no more than 109 Ohm. It is more accessible to use metal casings, however, the casing material must be frictionally safe, as stated in Section 8 of GOST R 51330.0-99 — «Materials containing light metals used for the manufacture of casings of electrical equipment of groups I and II must ensure frictional spark safety.»
The mechanical strength of the casing is not the least important factor, the test methods for which are described in paragraph 23.4.3 of GOST R 51330.0-99.
Impact strength tests
Thus, in terms of the sum of the requirements, it is quite difficult to use plastic shells in the design of explosion-proof redundant IE.
Since it is desirable that the ES could be used in different temperature classes, ideally in temperature class T6, this imposes a limitation on the electrical design of the source: a pulse ES is preferable to a transformer one, since the heating of the device is less, therefore it is possible to use the ES shell as a radiator, simultaneously solving the problem of the device's tightness.
Special requirements are also imposed on the battery in terms of explosion protection. If the ES uses explosion protection type d — «explosion-proof shell», then in the event of a possible explosion of the battery, the outer shell of the ES must withstand the explosive impact.
Explosion protection type e p. 5.7 GOST R 51330.8-99 «Protection type e» defines other approaches. If we summarize the requirements for the battery that must be ensured for explosion protection type e, they come down to the following:
use a sealed battery;
apply a compound fill to electrical parts and connections to prevent possible gel splashing from the battery if it is damaged;
ensure protection of the device from battery overheating in the event of a dead short circuit on the battery contacts.
In a hazardous area, the installation of a redundant IE, its connection, laying of cables and wires, as well as grounding of the device should be carried out in accordance with the requirements of the project and the Electrical Installation Code. The types of wires and cables, as well as the method of their laying, are selected based on the class of the hazardous area.
According to the Electrical Installation Code (edition 6, chapter 7.3.), in hazardous areas of any class, it is allowed to use wires with rubber and PVC insulation; cables with rubber, PVC and paper insulation in rubber, PVC and metal sheaths.
In hazardous areas of any class, the use of wires and cables with polyethylene insulation and sheath is not allowed.
In hazardous areas of classes B-I and B-Ia, wires and cables with copper cores must be used. The use of wires and cables with aluminum cores is permitted in explosion-hazardous areas of classes: B-Iб, B-Iг, B-II, B-IIа.
Cable entries must be made using specialized input devices, and the entry points must be sealed. The corresponding requirements for sealing pipes, cables, wires are imposed when the cable route passes from an explosion-hazardous area to an area with a different hazard class or to an explosion-proof area.