Standards such as BS EN 61439-2, while ultimately beneficial to electrical designers and industry overall, can sometimes be confusing to the uninitiated. Here Andy Evans technical executive at Gambica, reports on the Controlgear Group Technical Committee’s (CGTC) view on how the standard applies to those distribution boards known as ‘panel boards’
Concerns have been raised as to whether the casing around a switching contact mechanism can constitute a Form 4 enclosure as defined in Annex NA of BS EN 61439-2 and thus achieve a particular standard of separation between functional units.
Panel boards are a type of distribution board, commonly consisting of a number of outgoing moulded case circuit breakers (MCCBs) or fuse switches, connected to a common busbar which in turn is fed from a single incoming MCCB. The outgoing connection can come from the MCCB device itself or onto a set of outgoing terminals associated with each outgoer. The arrangements made for the outgoing connections are many and various and have a big influence on the final Form of Separation.
The starting point for switchgear design is the assumption the equipment must be safe to use for anyone who will have access to it during its lifecycle. This includes the fitters, engineers, maintenance personnel and machine operators as well as other people who shouldn’t touch the equipment but conceivably could, such as passers-by.
Annex NA to BS EN 61439-2 defines the performance criteria for an assembly to Form 4 as follows:
Separation of busbars from functional units and separation of all functional units from one another, including the terminals for external conductors, which are an integral part of the functional unit.
Sub Criteria, Form 4a (Types 1-3)
Terminals for external conductors (are) in the same compartment as associated functional unit.
Sub Criteria, Form 4b (Types 4 – 7)
Terminals for external conductors (are) NOT in the same compartment as associated functional unit, but in individual separate, enclosed, protective spaces or compartments.
In order to apply these definitions, one has to answer the question, ‘What constitutes a functional unit and how is the necessary separation, as defined in the criteria above, created?’
The answer to this question is also provided in BS EN 61439-2, where a functional unit is defined as “A part of an assembly comprising all the electrical and mechanical elements that contribute to the fulfilment of the same function”.
Although alternative interpretations are sometimes given, BS EN 61439-2 actually states that the integral housing of a device, for example a moulded case circuit breaker, is sufficient to satisfy the separation requirements as follows:
8.101 Internal separation of PSC-ASSEMBLIES (power switchgear and controlgear assemblies)
Typical arrangements of internal separation by barriers or partitions are described in Table 104 and are classified as forms (for examples, see Annex AA).
The form of separation and higher degrees of protection shall be the subject of an agreement between assembly manufacturer and user.
PSC-assemblies can be divided to attain one or more of the following conditions between functional units, separate compartments or enclosed protected spaces:
- protection against contact with hazardous parts. The degree of protection shall be at least IP XXB;
- protection against the passage of solid foreign bodies. The degree of protection shall be at least IP 2X.
Note: The degree of protection IP 2X covers the degree of protection IP XXB.
Separation may be achieved by means of partitions or barriers (metallic or non-metallic), insulation of live parts or the integral housing of a device e.g. a moulded case circuit breaker.
It should be noted the Form of Separation is one of the design aspects that is ‘subject to agreement between manufacturer and user’.
So, to satisfy the main criteria for Form 4, one alternative is to merely use an MCCB which by definition has a moulded case enclosing the electrical and mechanical parts necessary for it to fulfil its function. In this case, the terminal compartment may also physically form one of the constructional elements of the MCCB device.
To effect this arrangement, a means of shrouding the terminals and connected cable glands to ensure a minimum of IPXXB is necessary. Form 4 Type 5 indicates this may be done by use of insulated coverings. Forms 4 Type 6 and Type 7 require the separation via metallic or non-metallic rigid barriers or partitions.
So, again, a suitably designed MCCB device can satisfy both the main criteria, for Form 4 and the sub-criteria for Form 4b, and depending on the materials used to form the termination chamber, can provide Form 4 Type 5 or 6 arrangements.
One key issue to note is neutral (N) conductors, as they contribute to the fulfilment of the same function, form part of a particular functional unit and, in respect of Forms of Separation, must be treated as part of the functional unit. To this end, each outgoing way must have its own individual N connection, usually alongside the phase connections, and not be connected at a common N bar or terminal.
For four pole functional units, this is not normally an issue but in the case of a TP&N system, it’s a little more complicated. It is usual for a triple pole MCCB, for example, to have a separable neutral link mounted immediately adjacent to the MCCB to allow connection of all external cables in the same protected space, assuming adequate shrouding of all four terminals. For this arrangement to remain within the definitions of a functional unit and separation, multiple components should be logically arranged without gaps so that they are readily seen as being within one space.
A common N termination point arrangement cannot be deemed to be Form 4 as there is no separation of the terminals for external N connections for each functional unit in this case.
There is no distinction in BS EN 61439-2 between a Form 4 declaration where MCCB enclosures are used to define separation of functional units in a single enclosure compartment and that employing MCCB devices mounted in separate compartments of a multi-compartment PSC- assembly. Both can be declared Form 4 separation and both meet the performance requirements for separation. However, separation is not the only criterion to be considered. Regardless of the form of separation employed or how it is achieved, all assemblies must meet all the other safety and performance criteria laid down in the standard, for example; short- circuit including emissions from devices, temperature rise, and protection against electric shock.
BS EN 61439-2 gives only typical arrangements of internal separation; fundamentally the objectives of the separation and how it is achieved is a matter for agreement between the customer and the manufacturer. As a result, the customer should give careful consideration to the needs of his application, for example maintenance requirements.
Gambica is the trade association for instrumentation, control, automation and laboratory technology in the UK. It has a membership of over 200 companies including major multinationals in the sector and a significant number of smaller and medium sized companies.
It covers the following five principal sectors of the
- Industrial automation products and systems
- Process measurement and control equipment and systems
- Environmental analysis and monitoring equipment
- Laboratory Technology
- Test and measurement equipment for electrical and electronic industries
Permission to reproduce extracts from BS EN 61439-2 is granted by BSI. British Standards can be obtained in PDF or hard copy formats from the BSI online shop: www.bsigroup.com/Shop.