How to select the best managed DC load distribution panel

Safe tower sites are vital for both the system, the techs who visit and the public. Proper selection is critical.

DC load distribution panels are an integral part of safe, efficient communications site designs. Multiple loads can be connected to allow a DC power supply to distribute power to up to 12 devices such as radios, repeaters, switches and links. When ICT pioneered the use of TCP/IP ethernet to add remote monitoring of each connected load, as well as the ability to power cycle each output remotely, it opened up a new range of benefits to tower site designers and managers by reducing the need to visit the site to troubleshoot, or power cycle a locked up device.

How to choose the right panel

There are a number of considerations before choosing the right load panel for any application. Total system current and individual load current needs; single- or dual-bus design; positive or negative ground (or both); and what type of overcurrent protection is required to protect not only the site, but worker safety as well.

Polarity

Most 12 and 24 VDC systems operate with a negative ground. If the entire system is the same polarity, then a single bus load panel may be the best choice. Some sites, however, will operate with only a positive ground; therefore, it is important to check to see if the load panel being considered supports the type of ground you need.

More frequently, sites are starting to mix DC voltages and polarities, such as when negative ground 12 VDC repeaters are used along with positive ground (-48 VDC) backhaul radios at the same site. In this case it is not only essential, but far more cost-effective, to utilise a dual bus load panel that can support two different voltages and polarities simultaneously.

Sizing

When designing a system, careful consideration must be given to not only the total system current the load panel is required to handle, but also the individual output load requirements.

Some panels may offer relatively high individual output load ratings, say 25 amps, but if the total system current rating is only 100 amps then only four outputs would be usable. Check to see if higher rated outputs can be provided. A 40-amp output is useful for higher loads, such as RF amplifiers; higher power requirements can be sourced suited to applications like LTE radios.

Safety

The main function of a fuse or circuit breaker is to protect conductors and equipment from damaging overcurrents and quickly de-energise faulted circuits minimising hazards to personnel. The absence of such a device could result in dangerous conditions, either as a result of heat build-up during an overload condition or to employees who are not able to visually confirm a circuit has been de-energised before working on it.

Standard Electrical Safety guidelines from organisations like NFPA, IEEE, ANSI, NEMA and many building codes provide clear direction regarding safety of electrical device installation and maintenance, including:

• utilise fuses (or breakers) with blown fuse (or breaker) indication to minimise exposure to energised components while troubleshooting the circuit
• provide selective coordination (only the area where the fault occurs is shut off)
• provide a system that is safe to service and maintain.
   

OSHA 29 CFR 1910.334(b)(2) states: “Reclosing circuits after protective device operation — after a circuit is de-energised by a circuit protective device, the circuit may not be manually re-energised until it has been determined that the equipment and circuit can be safely re-energised.”

Almost all DC load distribution panels in the market, including ICT’s distribution series product line, utilise fuses or breakers in accordance with safety guidelines issued by these organisations.

When a fuse or circuit breaker trips, it is highly advisable to visit the site to determine what the source of the overcurrent event was, which is something that is not possible to do remotely.

Selection considerations

Factor	Importance	Types available	Check
Bus design	Single bus is ideal for applications that require all the devices use the same voltage and polarity. Dual bus allows for different voltages and polarities on the same panel.	single dual single and dual	May offer only one model of single- or dual-bus design, with limitations on which polarity can be used.
Polarity	Sites can be designed to operate with a negative or positive ground, or sometimes both, depending on the type of equipment being installed.	single-bus models for + or − grounds dual-bus models for + or − polarities on same panel	May offer negative ground only, not suitable for 48 VDC systems.
Current rating	Both the individual output rating and total system rating must be considered when selecting a load distribution panel.	single-bus models with individual output ratings up to 40 A dual-bus models with individual output ratings up to 25 A	Peak output rating compared to system requirements.
Safety	Power systems design conventions include the use of mechanical safety devices such as fuses or circuit breakers to prevent a device failure from turning into a more serious problem such as overheating or burning of wire insulation. They also provide a level of safety for employees by ensuring circuits are de-energised before conducting any work on the circuits.	fuses or circuit breakers	Any incorporated mechanical failsafe device.
Added functionality	When considering choice of panel, determine what additional features and benefits are provided.	digital inputs to connect site monitoring sensors