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16 V is i t o u r w e b s i t e a t : w w w . e l e c t r ic a lr e v i e w . c o . u k

T E S T A N D M E A S U R E M E N T

Electrical Review • Vol 238 No 5

Unless organisations instigate a proper testing and maintenance programme, they will

only know if their lightning protection system is working properly when they suffer a

strike. By that time they could have suffered catastrophic damage to their buildings

and business, says Mike Henshaw, managing director at Omega Red Group

Lightning protection – test before a strike

M

ost building services would simply

not function correctly if faults or

defects were present but the correct

operation of a lightning protectionsystem only becomes obvious when it is called

upon to protect a structure. For this reason it is

even more vital to ensure that fully trained and

accredited engineers undertake regular testing

and maintenance works on vulnerable

structures and sites. The current in a lightning

strike is likely to be in the range of 2,000 -

200,000A and so an effective operational

system is vital to ensure the protection of assets.

The vast majority of structures in the UK use

BS6651 to inform their design, testing and

maintenance works in relation to lightning

protection. This standard states a “ competent

person” should carry out inspections so a good

rule of thumb is to look for contractors with

third-party accreditation of their ability to

design and report on lightning protectionssystems, accreditation such as that provided by

Atlas (Association of Technical Lightning and

Access Specialists).

BS6651 covers all aspects of Lightning

Protection but sections 31-34 are of par ticular

relevance for testing and maintenance.

As large parts of the lightning protection

system may be hidden or inaccessible after

completion, it is par ticularly important , and

indeed a requirement of the code, that each

component of a lightning protection system

should be inspected during the construction

stages of an installation. Special attention must

be given to any part of the system that will be

concealed upon completion. These components

may be hidden for aesthetic reasons or the

component may be an integral part of the

structure.Inspections should be carried out not on ly

during the installation process but also upon

completion and at regular intervals thereafter.

Figure one shows damage that has been

identified through regular inspections. The first

picture shows the conductor has been bent into

an ‘s’ shape next to the clamp. This ‘s’ would

create inductance during any further lightning

current flow and may result in a flashover from

the conductor to adjacent conductive parts,

which could cause fire or other undesirablemechanical effects.

The second picture shows loose tapes,

probably caused by the mechanical effects of a

lightning strike dislodging poorly fitted fixings.

Further strikes would cause a whiplash effect

on the tape and may damage further fixings or

rip the conductor away from the system

completely, thus leaving it incomplete.

Visual inspection of an installation should

take into account the following key points and

observations recorded in the detailed inspection

report:

• inspections should be repeated at fixed

intervals, preferably not exceeding 12

months. If the intervals are fixed at 11

months, the system will have been inspected

throughout every season of the year over aperiod of 11 years

• the mechanical condition of all conductors,

bonds, joints and earth electrodes should be

checked and any observations noted

• if a part is unable to be inspected, this should

be noted

• the bonding of any recently installed/addedservices should be checked.

This section deals with testing the earth

electrodes on the system, although reference is

made to a visual or measured test of any joints

or bonds. In practice, it is usual for inspections

of components to be undertaken rather than

for testing to be carried out.

Electrode testing requires experience and

expertise to ensure that any test carried out is

meaningful and reflects the resistance of the

electrode under test. Too frequently, Omega is

handed client information presenting resistance

readings that are obviously continuity tests and

not true earth-resistance tests.

There are two appropriate methods of

testing lightning protection earths: ‘Fall of

Potential/the 61.8% method’ and ‘Dead Earth’.‘Fall of Potential’ is the recommended

method and involves the electrode under test;

two reference electrodes, a set of leads and a

four-pole test meter. The electrode under test is

isolated and connected to the meter as shown

in figure two for the ‘Fall of Potential’ or figure

three for the ‘61.8%’ method. In turn, the test

meter is connected to the two reference

electrodes, which are driven approximately

300mm into the ground and located typically

25 and 50 metres away from the electrode

under test.

A test is made and the direct resistance of

the electrode under test is recorded on the

meter. This method, however, is only practical

if the electrode to be tested is located adjacent

to virgin ground where test electrodes can be

driven. In reality, in town and city centres forexample, this is very often not the case. The

presence of buried services and pipes may also

have an influence on the test current and the

final test value may be corrupted as a result of

these external influences. Reference electrodes

should therefore be sited away from such

potential disturbances.Fig.1 Damage that has been identifiedthrough regular inspections

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18 V is i t o u r w e b s i t e a t : w w w . e l e c t r ic a lr e v i e w . c o . u k

T E S T A N D M E A S U R E M E N T

Electrical Review • Vol 238 No 5

Where practical conditions dictate that the

‘Fall of Potential’ method cannot be used, the

‘Dead/Known Earth’ method is really the only

practical alternative. However, it is important

to be aware this method is open to error and

misrepresentation if the test engineer is notcompetent to determine an appropriate dead

earth or interpret the readings, which is why it

is essential to use an Atlas accredited engineer

to undertake tests of this nature.

The ‘dead earth’ can be any low-resistance

earth not directly or fortuitously connected to

the earth under test. A connection is made

from a suitable earth to the test meter, which is

in turn connected to the electrode under test as

in figure four, which shows the lightning

protection system acting as the known

‘dead/known’ earth. A reading is then taken

and the ohmic value achieved is effectively the

series resistance of the electrode under test and

the dead earth.

The ‘Dead Earth’ method has some

advantages when using the lightning protection

system as the low-resistance ‘dead/known’

earth, as, due to the equipotential bonding

required to other incoming services, it should

provide a low-resistance earth path. Testclamps, or the clamp to the rod in the

inspection pit, should be opened and the meter

connected to the rod/rod side of the test clamp

and the other side of the test meter connected

to the system side of the test clamp.

A reading can then be taken, which will

show the series resistance of the electrode under

test and the rest of the system together with

other connected parallel electrical and other

earth paths.

As these other parallel paths usually have a

relatively low combined resistance, the meter

reading is effectively the resistance of the

electrode under test as, if correctly selected, the

‘dead’ earth that is used is normally of such

low value that it has little impact on the final

result.

In addition to providing an ohmic value forthe electrode under test, this method also

verifies the circuit to the dead earth source and

by virtue of this, the electrical condition of the

joints in the system. If the connections from the

top of the test clamp to the air termination

through to the other earths on the system and

other parallel paths were loose or damaged,

they would provide a high

resistance, which the meter

reading would reflect. This

situation should then be

investigated so that any

high-resistance joints canbe addressed.

Where no access to an

electrode is possible and,

for example, the pile

foundations have been

utilised as the earth

termination, it is recommended that individual

reference rods are installed around the structure

and tested upon completion. These do not

necessarily form a part of the

installation but may be used as

comparisons against the

original pile foundation test

results. In short, if the reference

rod values have not increased

year on year then it can be

assumed neither has the

resistance of the pilefoundations.

The ‘Dead/known earth’ test

method also applies to clamp-

on CT type testers where

disconnection is not required,

although this type of testing is not always

practical.

At least two types of test are recommended,

one for each of the individual electrodes in

isolation and a second for a combined value.

The requirements of BS6651 are an overall

system resistance (excluding bonding to any

services) of 10Ω and each electrode not

exceeding 10 times the number of earth

electrodes on the system.

Any disconnection of the system should be

preceded with a test to ensure that it is not ‘live’and no testing should be carried out dur ing

storm conditions.

Failure to keep up to date, accurate records

can result in hidden parts of a system not being

adequately attended to and potentially

unnecessary remedial works being proposed

and executed, as a full assessment of the

installation has not been made. At the time of

the annual test and inspection, the following

records are needed either on site or in an

accessible place.

BS6651 states that the following records

should be kept:

• drawings of the lightning protection

system

• details of the geology (nature of the soil

and details of any special earthing

arrangements)• type and position of the earth electrodes

• test conditions and results obtained

• details of any alterations to the system,

including additions and repairs

• the name of the person responsible for

the system.

In order to comply with the

Construction Design and M anagement

Regulations, these records should be provided

at the completion of t he original installation for

inclusion in the project Health and Safety file.

The person responsible for the upkeep of the

building should recover the lightning protection

system records from this file and present them

to the engineer undertaking the first post-

installation inspection and test. Details of the

inspections should be recorded so tha t the

required information can be updated and

maintained. The programme of tests andinspections will identify what, if any,

maintenance is needed. BS6651 states that

attention should be given to the following:

• earthing

• evidence of corrosion or conditions likely to

lead to corrosion

• alterations and additions to the structure that

may affect the lightning protection system

(e.g. changes in the use of the building, the

installation of crane tracks, erection of radio

and television aerials).

Statistics show the UK alone is subjected to

around two million strikes per year and, in

order to ensure your lightning protection

system is operational when called upon,

bearing in mind you have no way of

determining when that may be, anymaintenance work should be carried out with

appropriate expediency.

In the hands of experienced engineers,

proper testing and maintenance of lightning

protection systems can become a routine, but

very necessary, part of a comprehensive safety

programme. At the very least the consequences

of not taking a thorough approach could incur

unnecessary costs but, given the destructive

potential of a lightning strike, those

consequences could be much worse.

Fig.3 61. 8% Method Test

Fig.2 Fall of Potential Test

Fig.4 Dead/Known Earth Test