RISK ASSESSMENT

According to section 443.4 of the 18th edition UK wiring regulations (BS 7671:2018) . . .

“Protection against transient overvoltages shall be provided where the consequence caused by overvoltage could:

(i) result in serious injury to, or loss of, human life, or

(ii) result in interruption of public services and/or damage to cultural heritage, or

(iii) result in interruption of commercial or industrial activity, or

(iv) affect a large number of co-located individuals.

For all other cases, a risk assessment according to Regulation 443.5 shall be performed in order to determine if protection against transient overvoltages is required. If the risk assessment is not performed, the electrical installation shall be provided with protection against transient overvoltages, except for single dwelling units where the total value of the installation and equipment therein does not justify such protection. Protection against switching overvoltages shall be considered in the case of equipment likely to produce switching overvoltages or disturbances exceeding the values according to the overvoltage category of the installation eg where an LV generator supplies the installation or where inductive or capacitive loads (eg motors, transformers, capacitor banks), storage units or high-current loads are installed.”

INDEX

RISK ASSESSMENT AS DETAILED IN BS 7671:2018

In order to understand whether surge protection is required in cases not fitting those outlined above, a risk assessment should be carried out to determine the calculated risk level. In the event a risk assessment is not undertaken, surge protection is required to be fitted.

The risk level is calculated using the formula outlined in section 443.5 of the wiring regulations (see below).

1. Environmental factors are given:

Environmentfenv
Rural and suburban85
Urban850
surge damage

Calculate the power cable length (BS 7671:2018 figure 44.3)

2. Calculate the power cable length (Lp) in km, as shown in figure 44.3 (BS 7671:2018) (if any lengths are unknown, assume that LPAL completes the total length up to 1km)

3. Find the lightning flash density (Ng) from figure 44.2 in the wiring regulations

4. Put the above values into the formula: CRL = fenv/(LpxNg) – If CRL < 1000, surge protection is required to be installed.

For example:

Building in an rural environment
Ground flash density (Ng) = 1
Environmental factor (fenv) = 85
Power cable length (Lp) = 2LPAL + LPCL + 0.4LPAH + 0.2LPCH
LPAL = assumed 0.6km
LPCL = 0.2km
LPAH = 0.2km
LPCH = unknown
= (2 x 0.6) + 0.2 + (0.4 x 0.2) = 1.48
CRL = fenv/(LpxNg) = 85/(1.48 x 1) = 57.4
CRL < 1000, so surge protection shall be installed

Building in an urban environment (supplied by overhead lines)
Ground flash density (Ng) = 1
Environmental factor (fenv) = 850
Power cable length (Lp) = 2LPAL + LPCL + 0.4LPAH + 0.2LPCH
LPAL = assumed 0.7km
LPCL = unknown
LPAH = 0.3km
LPCH = unknown
= (2 x 0.7) + (0.4 x 0.3) = 1.52
CRL = fenv/(LpxNg) = 850/(1.52 x 1) = 559.2

CRL < 1000, so surge protection shall be installed