Hi, You'll possibly find the information you need or guidane at least on National Grids website. Look for their National Safety Instruction (NSI) downloads. If the information you need is not there base your measurements on HSE's guidelines. HSG85 (I think) may assist.

Carrickman, Whilst the info from DNW is very good, please remember that you are only working with Low Voltage systems, much of the National Grid systems are High Voltage. If you are considering working on High Voltage systems then you need a Senior Authorised Person (SAP), this is covered in these guidance documents, however, unless you are in very heavy industry, working for a DNO or in other areas where you have HV distribution, then it is doubtful that much of the NG data will apply. If you are in a HV environment then you must contact your SAP to discuss this. Also and finally, HSG85 will be a better reference (I think also as I can't remember the number!) there will be more information out in July when BS7671 Amd 1 is issued as I gather this will be covered in this for low voltage fixed electrical installations in buildings. For machinery there is a whole different set of standards and the risks are very different from mains supplies for several reasons, not least of which is reduced potential energy due to a multitude of things!

DNW, I never stated the the NG NSI's did not cover LV. DNO rules may vary. I appreciate that the advice was offered as guidance. I am fully aware of the voltage bands and I think you will dind that there is a level at which a voltage is classed as non lethal and it's not 24V d.c. Any live working must be done safely, it is something I do regularly on LV systems and it is unavoidable. However, is the OP relating to NG/DNO type distribution, is it relating to installations covered by BS7671, or more to do with stuff covered by BS EN 60204? The level of energy available could be several magnitudes different, or, it could be the same (almost) throught. I will add, that I don't work on NG or DNO systems as this is their domain, nor do I do HV stuff, but I do LOTS of LV to 7671 & 60204, both dead and live regulary and the live side cannot always be avoided. However, the energy available from a 100V a.c. supply from a 100 VA transformer through a 1A mcb is several magnitudes less than that which is even available at a domestic incoming supply say at the electricity meter. There is still significant danger, however, there are differences in the nature of the supplies that must be considered.