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A guide to HVAC

Does your HVAC installation comply with Electricity at Work Regulations?

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HVAC (Heating, Ventilation, and Air Conditioning) refers to technology of indoor and automotive environmental comfort. HVAC system design is a major subdiscipline of mechanical engineering, based on the principles of thermodynamics, fluid mechanics, and heat transfer. Refrigeration is sometimes added to the abbreviation, to create HVAC&R or HVACR, or ventilating is dropped as in HACR (such as the designation of HACR-rated circuit breakers). HVAC is important in the design of medium to large industrial areas and offices where safe and healthy building conditions are regulated with respect to temperature and humidity, using fresh air from outdoors.

HVAC systems can provide ventilation, reduce air infiltration, and maintain pressure relationships between spaces. How air is delivered to, and removed from spaces is known as room air distribution. The starting point in carrying out a heat estimate for cooling and heating will depend on the ambient and inside conditions specified.

However, before taking-up the heat load calculation, it is necessary to find fresh air requirements for each area in detail, as pressurisation is an important consideration. In modern buildings the design, installation, and control systems of these functions are integrated into one or more HVAC systems.

For very small buildings, contractors normally ‘size’ and select HVAC systems and equipment. For larger buildings, designers and engineers, such as mechanical, architectural, or building services engineers analyse, design, and specify the HVAC systems, and mechanical contractors build and commission them.

The HVAC industry is a worldwide enterprise, with roles including operation and maintenance, system design and construction, equipment manufacturing and sales, and in education and research. The HVAC industry was historically regulated by the manufacturers of HVAC equipment, but Regulating and Standards organisations such as HARDI, ASHRAE, SMACNA, ACCA, Uniform Mechanical Code, International Mechanical Code, and AMCA have been established to support the industry and encourage high standards and achievement.

HVAC Equipment containing 3 phase frequency inverters (VSD) for fan speed control can affect the operation of RCDs installed in other parts of the installation, resulting in increased risk to staff from electrocution or fire hazards.

Figure 1 is based on the recommendations of EN50178 detailing the correct division of circuits containing VSDs and is covered by regulation 314 and 331 in the latest edition of the IET Regulations.

Type ‘A’ RCCBs should not be used for this application, due to the smooth DC residual currents that flow under certain fault conditions. Using a Type B RCCB for this application is a fundamental safety requirement recognised in EN standards to ensure that DC earth fault currents are detected and not allowed to circulate within the installation.

Electricity at Work Regulations (EWR)
EWR 4 places a duty of care on the Duty Holders for the Site, with regard to the Electrical Systems and the components making up that system: 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.

RCCB protection with VSDs
To comply with Regulation 4(1) 65 where the precautions taken include an RCCB to reduce the risk of death or injury (Regulation 8), the RCCB must be suitable for use with VSDs. In order to comply with current laws on H&S, the Duty Holder must make reference to the VSD manufacturers recommendations relating to the type and characteristics of the RCCB required. If this is not clearly stated in the operating instructions, it is imperative to obtain the VSD manufactures recommendations in writing. Depending on the topology of the VSD, and certainly with 3 phase units, it would not be safe to use an AC or A Type RCCB.

AC operational leakage currents
Typically, 3 Phase Inverters are used for speed control and the associated EMC filters and motor cables, and they generate leakage currents at a nominal supply frequency (50Hz) and at various harmonic frequencies. Leakage currents in the higher frequency ranges can be significant and from a safety perspective cannot be ignored.

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.

Figure 2 gives an example of the frequency range of various leakage currents present in a system containing a 32 amp VSD. At 50Hz the leakage current is less than 3mA, however the actual maximum leakage current occurs at 7815Hz and is approaching 2000mA. This installation had to be taken out of service by the user and the Drives Manufacturer was contacted in order to look at ways to reduce the leakage currents so as to meet fire protection requirements of 300mA for this site.

Fortunately for the user, the standard RCCB they tried to use in the installation tripped on start-up due to the inrush current associated with the capacitance of the EMC filter (Doepke Type B RCCBs withstand 3 kA - 8/20µS pulse or 5 kA -8/20µS for Selective version). Also, it is worth remembering that standard RCCBs are only designed to operate at the nominal supply frequency; for example 50Hz in the UK. The commissioning engineer should have access to a residual current analyser to gain a complete picture of the installation’s leakage currents to check the suitability of the protection devices required for the installation.

DC Residual Currents
Under certain fault conditions, 3 phase inverters produce smooth DC residual currents (as shown in figure 3) and this current cannot be detected by conventional RCCBs. In addition, this DC current – if allowed to circulate within the installation – would produce magnetic saturation within the AC trip coil of the conventional RCCB or RCBO. This means that the device would not be able to sense AC leakage currents. A product applied outside of its original design tolerance cannot be and should not be relied on to perform a safety function (IET wiring regulation clause 133.1.3).

Safe application of HVAC VSDs using B Type RCCBs
The Electricity at Work Regulations makes reference to the IET Regulations as a guidance document (unless the site is specifically covered by another code of practice such as Mines & Quarries).

The existing Regulation 331.1 is quite clear in its requirement and states: ‘An assessment shall be made of any characteristic of equipment likely to have harmful effects upon other electrical equipment.’

A simple assessment carried out under Regulation 331.1 in accordance with existing Health & Safety legislation on a system containing VSDs and requiring RCCB protection will quickly identify that a conventional RCCB cannot be used.

Only B Type RCCBs can be used safely with VSDs incorporating the appropriate trip characteristic, compatible with the operational and safety requirements of the installation for People and or Fire protection. To safely commission an installation containing a VSD, it is important to always follow the manufacturers’ instructions. A residual current analyser (such as DRCA1) can be used to determine the actual maximum level and frequency of the leakage currents present in the installation. Having this information available will enable the engineer to validate the RCCB selection, reducing the risk of nuisance trips, but more importantly, also make certain that the RCCB is correctly selected for the safety requirements associated with the installation.

Chaz Andrews is Technical Manager at Doepke UK Ltd.

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