Updating HVAC equipment: Electrical installation safety

24Mar

Figure 1 Single phase split air conditioning unit or multi-split units

Chaz Andrews from Doepke UK discusses the safety implications of using existing circuits to feed new electrical equipment?

Unplanned replacement of electrical equipment puts management and contractors under pressure to find speedy solutions.

From 2015, users can still legally operate old R22 based HVAC systems providing they are safe, but the complete ban on the use of R22 in the maintenance and repair of existing installations is fast approaching. Choosing not to upgrade or replace before the deadline represents a significant business risk for many companies and commercial property owners.

Existing wiring

In 2012 17% of fires in commercial and public buildings were attributed to electrical distribution compared to 11% in 2001 - source UK National Fire Statistics. This equates to 8 electrical fires every day of 2012, putting employees and members of the public at risk. One possible reason for the increase in electrical fires could be the result of companies and duty holders cutting corners on electrical specification and maintenance.

The legal obligations for duty holders, contractors and equipment suppliers can be found in the appropriate Statutory Instruments. For example electrical equipment installation, maintenance and repair activity is covered by Electricity at Work Regulations 1989 (EWR). Contractors have a duty of care with regard to their own activity and how it impacts on others, as do those companies and duty holders employing a contractor to complete work on their site. Whilst the principle objective of the EWR’s relates to electricity in work activity, staff /contractors working near electrical equipment must not be put at risk of injury. The HSE EWR guidance helps clarify the principle of preventing injury at work, related to electrical equipment and installations based on the terms absolute and reasonably practicable.

Duty holders should not confuse their legal obligations “to prevent injury” detailed in the EWR Regulations and the minimum level of performance design and installation advise given in BS7671. As this standard covers both commercial and domestic premises, it must be applied taking into account the risk of the foreseeable “Injury” and what is considered to be “reasonably practicable” with regard to the design, installation and use.

Compliance with a British Standard cannot confer immunity from legal obligations. What may be considered acceptable in a domestic installation may not be acceptable in a commercial installation where the duty of care and insurance requirements are clear and proven by existing cases.

For example the installation of an externally mounted HVAC split unit on a business premises, without RCD protection may be possible within the recommendations given in BS7671. However, it is reasonable to foresee that an internal earth fault which is not sufficient to operate the short circuit protection device could result in a number of “injuries” which may be as a result of indirect contact due to a single fault. An electric shock could lead to nerve damage, burns, reaction injuries, falls, electrocution, fire: smoke, evacuation and burning. The relative cost of RCD protection is low compared to risk, making it reasonably practicable to fit RCD protection to reduce the risk.

Designers / installers do take risks by not reading the detail contained in safety instructions, relating to the requirements of the electrical installation for new equipment – see example given below. This is particularly important with regard to installing new HVAC equipment, as most manufactures of split systems (external mounted units) recommend the use of RCDs to reduce the risk of electric shock. The implications of this may not be immediately obvious by just making a quick reference to BS7671; see Reg. 133.2 – 4 and effects see Chap. 33 331.1 specifically vi to xi.

Where the installation is covered by Part P of the building regulations e.g. Buildings containing business premises and dwellings, replacing HVAC equipment would be classed as new work as at the very least the circuit's protective measures are affected - see below.

Practical example

These units contain inverter speed control for fans and compressors, which impact on the design of electrical installation. Check the detail given in the HVAC’s technical manual, relating to earthing and RCD protection. The protective conductor current must be confirmed to ensure that it does not exceed that allowed under Regulation 543.7.102, if the total protective conductor current for all equipment exceeds this value, then Regulation 543.7.103 will apply. The residual current associated with the HVAC solid state control and EMC protection affects the selection of the RCD.

The existing EN Safety Standard defines 3 types of RCD that could be used with inverter circuits similar to figure 1. The exact Type of RCD required will depend on the individual characteristics of the HVAC manufactures design and consequently has to be specified by the HVAC manufacturer to meet the essential safety requirements of the Machinery Regulations.

The DC content generated in the AC supply by an HVAC inverter is primarily determined by the capacitance and the voltage on the DC side of the bridge. The high frequency harmonic content is mainly associated with the switching frequency of the inverter plus all the associated components and cabling in the HVAC unit.

As a general guide the limits of operation are detailed below, however to meet the UK Regulations the machinery manufacturer should specify the “type” of RCD that can be used safely with their equipment.

Type A: Detecting AC residual currents and pulsating DC residual currents at mains frequency (50Hz) with a maximum smooth DC content of 6mA.

Type B: Detecting AC residual currents and pulsating DC residual currents at mains frequency (50Hz), smooth DC residual currents greater than 10mA and AC residual currents not equal to mains frequency. Minimum resistance to surge currents: 3kA 8/20µs pulse.

Type F: Detecting AC residual currents and pulsating DC residual currents at mains frequency and composite residual currents containing mains, low and high frequency components, with a maximum DC content of 10mA. They have a greater resistance to surge currents associated with inverter capacitance. Minimum 3kA 8/20µs pulse.

Ignorance cannot be used as a defence under Regulation 29 (EWR) and Regulation 4(4) specifically refers protective equipment being suitable for use. Regulation 4(1) 65: “The safety of a system depends upon the proper selection of all the electrical equipment in the system and the proper consideration of the inter-relationship between the individual items of equipment.

It is unsafe to connect HVAC equipment to circuits protected by standard RCDs. Type AC RCDs are not suitable for use in circuits containing inverters that generate pulsed and smooth DC residual currents and in some cases mixed frequency AC residual currents.
Doepke UK have a free 60 page guide available from WWW.doepke.co.uk.

Figure 1 Single phase split air conditioning unit or multi-split units