Doepke – HVAC equipment: Electrical installation safety
Electrical Distribution Faults – Making the right decision about existing wiring In 2012-13 20% of fires in non-residential premises, were attributed to faulty electrical equipment and their supply cables1. To see how easy it is for an electrical fire to start, refer to the bottom of Doepke Home Page “How RCDs prevent fire” www.doepke.co.uk Electrical specification, inspection and maintenance plays a key part in “Site Safety”, especially when planning to update or install new electrical equipment containing inverters.What the Regulations require
Contractors have a duty of care with regard to their own activity and how it impacts on others, as do those Companies 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. For example equipment producing high leakage currents installed on the outside of a building near a window, in the event of poor or faulty protective bonding, could pose a risk to staff cleaning the widows. The majority of manufactures of split systems (external mounted units) recommend the use of RCCBs to reduce the risk of electric shock.
High earth leakage currents
Elements of the Power Drive System (PDS) VSD / EMC filters / Motor cables / length / Motor and earthing all have an impact on the value of operational leakage current and EMC performance. Consequently the machine manufacture is responsible for providing sufficient detail to enable their equipment to be installed and operated safely. Designers and Installers do need to take note of the detail contained in safety instructions for existing or new equipment. The manufacture should clearly state the range of operational earth leakage currents that may be present under normal operation:
a) To select the appropriate protective conductor CSA and method of connection;
- Protective conductor current > 3.5mA < 10mA; see Regulation 522.214.171.124
- Protective conductor current ≥ 10mA see Regulation 5126.96.36.199
b) To select the RCCB Type & Sensitivity;
- Compatibility of characteristics see Regulation 331.1 and 341.1
Certain VSD / EMC filter combinations cannot be used with 30mA RCCBs, as the operational leakage current could exceed the trip threshold of the RCCB. As a general rule of thumb – the operational leakage current should not exceed 33% of RCCB sensitivity. The leakage current generated at 50Hz (the value normally quoted by the Inverter manufacture), will increase as function of the frequency for the various harmonics within the system- See Fig 2. Before purchasing any equipment, it is imperative to check with the HVAC manufacture on the Type of RCCB and the minimum sensitivity (in mA’s) that can be used with their equipment, to avoid nuisance tripping.
Example: Single phase Split Air Conditioning unit or Multi-split units
The individual characteristics of the HVAC equipment design determine the characteristics of the RCCB. For example if there is a step-up section after the filter, the increased dc voltage value will increase the smooth dc content of the residual current, consequently the Type of RCCB has to be specified by the HVAC manufacturer to meet the essential safety requirements of the Machinery Regulations.
Caution! Always use an earth leakage circuit breaker designed for inverter circuits to prevent faulty operation.
The existing EN Safety Standards defines 3 Types of RCCB that could be used with inverter circuits similar to that given in figure 3.
Single phase inverters producing 50Hz residual currents. (see PDS manufacturer’s recommendations)
Limits of operation
Sine wave or ½ wave @ 50Hz
Smooth dc residual current component < 6mA.
Capacitors in the inverter / EMC filter generate transients which can result in nuisance tripping.
Use of high immunity RDCs e.g. Type A KV have a greater resistance to transients.
High frequency harmonic currents could cause overheating and failure of the RCD trip circuit.
Single phase inverters producing composite residual currents containing mixed frequency components,
Limits of operation
Composite residual currents: > 10Hz < 1kHz
Smooth dc residual current component < 10mA
Built in 10mS delay for capacitive transients
Transient Peak < 3kA 8/20μs pulse.
3 phase inverters use only Type B or Single phase inverters exceeding operational limit for Type F
Limits of operation
Composite residual currents: >1kHz Refer to RCCB manufactures characteristics
Smooth dc residual component: Must trip if dc component > 2x ac IΔn for RCCB
Quoting the Electricity at Work Regulations; “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…” Refer to the HVAC Equipment Manufactures installation instructions relating to the type of earth fault protection that can be safely installed upstream of the HVAC equipment.
Chaz Andrews – Technical Manager, Doepke UK Ltd
Doepke UK have a free 60 page Technical Application Guide Available. Log on to www.doepke.co.uk to download the guide or obtain further information on RCCBs. For technical support please e mail firstname.lastname@example.org