400 Chapter 13
I would not be certain about the whole of Europe, but to the best of my knowledge it is the
same as the United Kingdom, that is, not balanced. The neutral line is at earth potential,
give or take a volt, and the live is 230 V above this. The three-phase 11-kV distribution to
substations is often described as “ balanced ” but this just means that power delivered by
each phase is kept as near equal as possible for the most effi cient use of the cables.
It has often occurred to me that balanced mains 115 V–0–115 V would be a lot safer.
Since I am one of those people that put their hands inside live equipment a lot, I do have a
kind of personal interest here.
13.4 Class I and Class II................................................................................................
Mains-powered equipment comes in two types: grounded and double insulated. These are
offi cially called Class I and Class II, respectively.
Class I equipment has its external metalwork grounded. Safety against electric shock is
provided by limiting the current the live connection can supply with a fuse. Therefore, if a
fault causes a short-circuit between live and metalwork, the fuse blows and the metalwork
remains at ground potential. A reasonably low resistance in the ground connection is
essential to guarantee the fuse blows. A three-core mains lead is mandatory. Two-core
IEC mains leads are designed so that they cannot be plugged into three-pin Class I
equipment. Class I mains transformers are tested to 1.5 kV rms.
Class II equipment is not grounded. Safety is maintained not by interrupting the supply
in case of a fault, but by preventing the fault happening in the fi rst place. Regulations
require double insulation and a generally high standard of construction to prevent
any possible connection between live and the chassis. A two-core IEC mains lead is
mandatory; it is not permitted to sell a three-core lead with a Class II product. This
would present no hazard in itself, but is presumably intended to prevent confusion as to
what kind of product is in use. Class II mains transformers are tested to 3 kV rms to give
greater confi dence against insulation breakdown.
Class II is often adopted in an attempt to avoid ground loops. Doing so eliminates the
possibility of major problems, at the expense of throwing away all hope of fi xing minor
ones. There is no way to prevent capacitance currents from the mains transformer fl owing
through the ground connections (see Section 13.3). It is also no longer possible to put a
grounded electrostatic screen between primary and secondary windings. This is serious