The largest-ever tunnelling project ever undertaken by the National Grid is currently burrowing its way under Barnet. But it is uncertain who is in charge of monitoring safety, writes JULIAN HILLS

Capable of housing two giant 400 kilovolt (kv) cables, the 20km 'London Connection' tunnel is engineering on a grand scale.

Running directly beneath the A5 Edgware Road between National Grid substations at Elstree and St John's Wood, it is designed to meet the growing demand for electricity in north-west London and the City.

Boring machines are knocking out a three-metre wide tunnel at depths of between 20 and 35 metres, with five tunnel access shafts at Elstree, Canons Corner, Colindale, Cricklewood and Kilburn, used for ventilation and holding equipment.

The depth and size of the tunnels has been designed so that streets will no longer have to be dug up to maintain and repair the cables, and so that at pavement level, the risk of exposure to dangerous levels of electric and magnetic fields is reduced.

Work on the London Connection project began in spring last year and is expected to be completed by spring 2005, with the first 400kv cable due to be installed next year.

Once operational, these cables will begin generating considerable magnetic fields, raising questions about the health of people living near the Edgware Road in Barnet.

Levels of electro-magnetic fields (EMFs) generated by the cables will be, according to National Grid EMF engineer Peter Hanney, less than those already found in most homes and well below national guidelines.

"The actual levels given off by cables will be so low at ground level, they probably won't be detectable," he said. "The fields generated from these cables, with both circuits running at 100 per cent, would have a maximum possible reading of less than 0.2 microteslas.

"The safe limit set by the National Radiological Protection Board (NRPB) is 1,600 microteslas."

EMF expert Professor Denis Henshaw, from Bristol University, said that that limit was far from safe, with the limit in Switzerland being set many hundreds of times lower, at one microtesla.

In fact, certain health conditions are associated with far lower readings miscarriage at 1.6 microteslas, childhood leukaemia at 0.4 and adult leukaemia at 0.2.

Nevertheless, at under 0.2 microteslas, the National Grid figures for the new cable should not give cause for concern.

"The deeper the burial of the tunnel the better," Professor Henshaw said. "As regards exposure limits and effects on health, you are looking at typical exposure limits from overhead cables of between 0.2 microteslas and 1.6 microteslas. But even at 0.4 microteslas you are doubling the risk of childhood leukaemia.

"It is down to the National Grid to publish what the surface levels are, and they have to consider maximum levels.

"The problem is that in the UK, there is hardly any regulation. The National Grid needs to be reassuring the public that exposure is below the limit of 0.2 microteslas."

A study undertaken in February 2001 by Professor Henshaw showed that conditions such as skin cancer and depression were likely to occur in people living near high-voltage overhead cables, even when EMF levels were relatively low.

It remains uncertain exactly who will be checking to make sure these low levels of radiation are maintained. There are no legal obligations to monitor radiation, just official guidelines.

A Barnet Council spokesman said its role was confined to planning matters concerning the surface level tunnel access shafts and had no role in monitoring EMF levels, which it said was down to the NRPB.

A spokeswoman for the NRPB said the National Grid was responsible for demonstrating the equipment complied with national guidelines, and bodies such as the NRPB would only be brought for an independent assessment if Barnet Council decided to employ them on a commercial basis.

"Our scientists confirm that the predicted levels do fall well below national and international guidelines," the NRPB spokeswoman said.

Myk Tucker, of Hillersden Avenue, just off the A5, said: "Irrespective of what the National Grid is saying, the jury is still out on this project.

"We will be out there with our meters doing our own assessments. The NRPB is talking about limits of 1,600 microteslas, but I think that is the level at which skin starts to cook. They do not accept there are any dangers associated with EMFs.

"There should be a full public debate about what is going on here, because no-one seems to know about it. Everyone says: what tunnel?'"

For the National Grid, Mr Hanney said: "We have gone through a massive public consultation exercise for this project.

"It is a big job. They are doing work at both ends, but at the moment, they are just digging up clay and soil."

For more information about the London Connection project, ring the National Grid community relations team on 0800 169 2392.

Professor Henshaw's department at Bristol University runs a web site at www.electric-fields.bris.ac.uk, holding information on EMFs.