Spencer Henry, Member of the Institute of Clerks of Works and Construction Inspectorate of GB, discusses the rules around chasing and placing of accessories within other approved documents.
Approved Document P and Approved Document M are not the only the instruments which refer to specific parts of electrical installation work, as I’ll explain in this editorial article.
While undertaking auditing and clerking of works, I have in the past identified particular issues on sites where chasing has been undertaken. More often than not vertical chases are acceptable, but horizontal chases tend to be too deep. Accessories are poorly located on adjoining walls, particularly framed or studded walls
Approved Document A for structure and Approved Document E for resistance to sound are often not referred to by electricians. Guidance on chases and the restrictions on locations for accessories are contained within these instruments and are often overlooked.
Approved Document A for structure Section 2C, applies to residential buildings up to three storeys, small single storey non-residential buildings and small annexes of residential buildings, such as garages and outbuildings. The document states that, vertical chases shouldn’t be deeper than 1/3 leaf of the wall, and horizontal chases shouldn’t be deeper than 1/6 of the leaf.
As an example, the typical depth of a block or brick leaf of a wall is between 100 and 102.5mm, meaning that the maximum depth of a vertical chase should be no more than 33 – 34mm and horizontally no deeper than 16 – 17mm.
It’s also important, particularly with hollow block walls, that chases are positioned so not to impair the structural stability. With any structure if you’re not sure or believe you may have inadvertently compromised the stability, consult a structural engineer. In most cases these issues can be rectified easily.
Approved Document E for resistance to sound applies to the construction of new buildings. There are some slightly different rules here for certain types of wall (see Fig 1):
Wall type 1: Solid masonry & Wall type 2: Cavity masonry
You cannot use deep sockets, deep chases or place accessories such as sockets back-to-back on these types of wall
Wall type 3: Masonry between independent panels
No restrictions are placed on this wall type; generally you wouldn’t require chasing or boxes cutting in due to the cavity space available between the masonry and the panels.
Wall type 4: Framed walls with absorbent material
Plasterboards are not to be chased. Accessories are not to be placed back-to-back, a minimum edge-to-edge stager of 150mm is recommended for this.
In all cases it is highly recommended that where any accessories forming part of the fabric of the building are installed, fire and sound barriers are reinstated by use of fire-rated acoustic cap inserts in accessory back boxes.
In conclusion, referral to these statutory guides during the construction will ensure correct and safe installation of electrical cables and accessories, while maintaining fire and acoustic ratings and structural stability.