118 1-800-447-2371 www.lutze.com LUTZE Technical Overview NFPA 79 Requirements for VFD Cables NFPA 79 Chapter 4 “General Requirements and Operating Conditions” describes the general requirements and conditions for the operation of the electrical equipment of the machine. The relevant section regarding VFD cable can be found in article 4.4.2.8 “Circuits Supplied from Power Conversion Equipment” which addresses the proper selection of insulation materials and/or cables to be used with power conversion equipment such as VFDs and servo drives. VFDs and servos utilizing Pulse Width Modulation (PWM) technology typically create power distortions leading to harmonics, voltage spikes and overcurrent issues. This section aims to bring awareness to a potential safety concern regarding the use of thermoplastic wiring such as PVC or PVC/Nylon commonly used in power and control tray cables which are not designed as VFD or motor supply cables under such conditions. NFPA 79 article 4.4.2.8 “Circuits Supplied from Power Conversion Equipment” describes two requirements to safely operate a VFD which is crucial for drives operating at 480V or higher. Conductors and equipment must be identified as suitable for the electrical power characteristics and in accordance with all manufacturer’s instructions. NFPA 79 provides additional guidance in annex A.12.1.1(5) which describes typical operating conditions that may cause some insulated circuit conductors (e.g., thermoplastic) to be susceptible to breakdown from arcing if considerations are not made to mitigate them. This includes the consideration of power conversion equipment output voltage, current, and frequency; as well as the length, dielectric strength and spacing of conductors. A safety concern may exist when thermoplastic wiring materials are being used and many drive manufacturers instructions specify against the use of such wiring. Most thermoplastic insulation types have difficulty to withstand the output voltages and currents from a VFD utilizing pulse width modulation over time. Thermoplastic insulation, such as PVC/Nylon which commonly used in power tray cables, can create problems, for example, in moist environments or in longer cable runs between VFD and motor. The dielectric properties of PVC cause high cable capacitance leading to high charging currents; the low voltage breakthrough resistance can lead to corona discharge and the potential for shorting out the cable. Additionally, thermoplastic PVC can melt and be deformed when exposed to excessive heat generated by short circuits or overloads. Insulation types “RHH, RHW, RHW-2, XHH, XHHW, or XHHW-2” all are thermoset Insulation types per UL 44 which have strong dielectric properties and will not melt. These are common designations translating as follows: XLPE Cross Linked Polyethylene is a thermoset insulation material RHH Rubber High Heat resistant RHW Rubber Heat and Water resistant RHW-2 Rubber Heat and Water resistant 90°C dry and 90°C wet locations XHH Crosslinked (Polyethylene) High Heat resistant XHHW Crosslinked (Polyethylene) High Heat and Water resistant XHHW-2 Crosslinked (Polyethylene) High Heat and Water resistant 90°C dry and 90°C wet locations Informational note: Even though the “R” stands for “Rubber”, the designation includes other thermoset materials such as XLPE, SBR, CPE and others. Designations such as THHN (Thermoplastic High Heat resistant, Nylon coated) or any designation beginning with T is considered thermoplastic material and should be avoided unless the equipment manufacturer specifically permits them. All products within the DRIVEFLEX® series are made with XLPE insulation of type XHHW-2 or RHW-2 depending on model. LUTZE Technical Overview NFPA 79 Requirements for VFD Cables NFPA 79 Chapter 4 “General Requirements and Operating Conditions” describes the general requirements and conditions for the operation of the electrical equipment of the machine. The relevant section regarding VFD cable can be found in article 4.4.2.8 “Circuits Supplied from Power Conversion Equipment” which addresses the proper selection of insulation materials and/or cables to be used with power conversion equipment such as VFDs and servo drives. VFDs and servos utilizing Pulse Width Modulation (PWM) technology typically create power distortions leading to harmonics, voltage spikes and overcurrent issues. This section aims to bring awareness to a potential safety concern regarding the use of thermoplastic wiring such as PVC or PVC/Nylon commonly used in power and control tray cables which are not designed as VFD or motor supply cables under such conditions. NFPA 79 article 4.4.2.8 “Circuits Supplied from Power Conversion Equipment” describes two requirements to safely operate a VFD which is crucial for drives operating at 480V or higher. Conductors and equipment must be identified as suitable for the electrical power characteristics and in accordance with all manufacturer’s instructions. NFPA 79 provides additional guidance in annex A.12.1.1(5) which describes typical operating conditions that may cause some insulated circuit conductors (e.g., thermoplastic) to be susceptible to breakdown from arcing if considerations are not made to mitigate them. This includes the consideration of power conversion equipment output voltage, current, and frequency; as well as the length, dielectric strength and spacing of conductors. A safety concern may exist when thermoplastic wiring materials are being used and many drive manufacturers instructions specify against the use of such wiring. Most thermoplastic insulation types have difficulty to withstand the output voltages and currents from a VFD utilizing pulse width modulation over time. Thermoplastic insulation, such as PVC/Nylon which commonly used in power tray cables, can create problems, for example, in moist environments or in longer cable runs between VFD and motor. The dielectric properties of PVC cause high cable capacitance leading to high charging currents; the low voltage breakthrough resistance can lead to corona discharge and the potential for shorting out the cable. Additionally, thermoplastic PVC can melt and be deformed when exposed to excessive heat generated by short circuits or overloads. Insulation types “RHH, RHW, RHW-2, XHH, XHHW, or XHHW-2” all are thermoset Insulation types per UL 44 which have strong dielectric properties and will not melt. These are common designations translating as follows: XLPE Cross Linked Polyethylene is a thermoset insulation material RHH Rubber High Heat resistant RHW Rubber Heat and Water resistant RHW-2 Rubber Heat and Water resistant 90°C dry and 90°C wet locations XHH Crosslinked (Polyethylene) High Heat resistant XHHW Crosslinked (Polyethylene) High Heat and Water resistant XHHW-2 Crosslinked (Polyethylene) High Heat and Water resistant 90°C dry and 90°C wet locations Informational note: Even though the “R” stands for “Rubber”, the designation includes other thermoset materials such as XLPE, SBR, CPE and others. Designations such as THHN (Thermoplastic High Heat resistant, Nylon coated) or any designation beginning with T is considered thermoplastic material and should be avoided unless the equipment manufacturer specifically permits them. All products within the DRIVEFLEX® series are made with XLPE insulation of type XHHW-2 or RHW-2 depending on model.
