Electrical Safety in Medium Voltage

Medium Voltage disconnectors in electrical installation s are not safety devices !

MV switch disconnector

Last June, I presented a paper at the CIRED conference with a title “Medium Voltage Disconnector s and Safety”… and a main message “MV disconnector s are not electrical safety devices”. The topic could be seen as controversial, but it actually raised some real concern about how the electrical safety could be addressed.

The attendants were a bit surprised by the message, as the confusion is very common. It is probably linked to the fact that, for LV installations, disconnection is set in electrical specifications as a safety function, with relevant requirements and acceptance criteria. However, it is not the case for devices above 1kV, because the earthing of the circuit is always required before any work could be performed on the conductors (except if live working procedures are applied).

When coming to electrical specification, our industry has an “ancient tendency”, a bad habit, which is to forget about the basic analysis of the application. Some speak about the “copy-paste” syndrome…

Such a basic analysis is required for an effective risk assessment, and then the proper definition of the protection measures to be applied. Using the word “safety” associated with a device which is not specified, nor used, with a global understanding of the application leads to an over-confidence which could be dangerous.

Safety is a system concern, and can’t be limited to using any so-called “safety device”.

Some discussion raised around my presentation

As most situations involving an operator also imply to have one side earthed, the open gap is in such cases not over-stressed, compared to any other insulation in the installation.

However, a special situation has been highlighted by a participant, about the cable test operation; in such a situation, the disconnector is actually used with two live circuits on its sides, and operators are possibly in the vicinity of the cables under test… In this case, the claim is that the disconnector becomes a “safety function”.

The fact is that such situation is not properly taken into consideration when specifying the disconnectors, because dielectic withstand under such situation is not defined nor assessed, and the possible failure mode is unknown.

Field operation has shown that, during such operations, available switchgear is safe, that the risk is low – Safety is defined as a tolerable risk level – but nevertheless, the need is not correctly analysed, and the requirements expressed for the disconnector s have no link with this cable test situation.

Keeping an electrical specification based on wrong assumptions could lead to dangerous situations, for which the electrical specification could be met, but the expected service would be impaired.

And trying to define safety rules on a device, without linking them to the way the device is installed and used, is not enough to achieve a safe operation. So, when the specification use the words “safety device”, the consequences of an over-focus, or over-confidence, could become critical.

Electrical risk does exist. It is better to understand it properly, in order to mitigate it and to prevent as far as possible the possible consequences.

We should train specifiers and operators of any electrical installation for a better understanding of the functions, performances and limitations of the available devices, and also to a better knowledge of the safety analysis ideas.

Any feedback is welcome
Didier Fulchiron

Article presented at CIRED : Paper presented at CIRED conference, with slide show below