Current--limiting switch

Details Current--limiting switch Current--limiting switch

1996-01-26

1999-01-12

Scott, J. R.

Electricity: electrothermally or thermally actuated switches

Electrothermally actuated switches

Expansible or vaporizable fluid-actuated

337117, 337417, H01H 8102, H01H 3772, H01H 3736, H05B 310

Patent

active

058595791

DESCRIPTION:

BRIEF SUMMARY
The invention relates to a current-limiting switch having current connections and contacts, of which one contact is a fixed contact and the other a mobile contact, and having an associated drive for opening the mobile contact when a predetermined electric current intensity is exceeded.
In power distribution networks, the current which flows when a short circuit occurs should be limited as much as possible in order to prevent damage to lines and loads since the only other way to prevent such damages is extreme overrating. Conventional mechanical low-voltage switches obtain the mechanical energy required for switching off from a spring energy store. In some cases, additional reinforcement is provided by the intrinsic magnetic field of the short-circuit current. The opening speed in such switches is limited by the complex mechanical construction on the one hand and the limited spring energy on the other. The current-limiting effect of such switches is thus restricted.
From the WO-A-91/12643 and the EP-A-0 487 920, current-limiting arrangements are known as series-connected devices for switches which specially exploit the PTC (positive temperature coefficient) effect. There, high-current resistors are used which consist essentially of a polyethylene layer filled with carbon black and exhibiting the PTC effect. In such a high-current resistor usable as a protective element, the polymer resistive member should be connected with its surface areas to an electrode in order to guarantee the PTC effect, a pressure device being present which exerts a pressure perpendicularly on the electrodes and the surface areas of the resistive member of the conductive polymer layer.
In contrast, it is the object of the invention to specify a current-limiting switch which works based on another physical principle.
This objective is solved according to the invention in a switch of the type stated in the introduction in that the drive for opening the mobile contact is a thermodynamic drive. With such a drive, it is possible to apply sufficient switching energies in a thermal manner.
In the invention, the contacts are arranged in a closed housing made of insulating material and a disk-shaped resistive member is arranged between the contacts. An expansion volume is preferably present between the resistive member and the mobile contact in the housing made of insulating material. The resistive member can consist of a plastic containing graphite, e.g., on the basis of polyethylene, or can be formed by a multitude of carbon fibers which are made into a film or felt-like consistency.
Unlike the prior art, the mechanical switching energy required for contact separation is thus applied electrothermally in the invention. For this purpose, the high-current discharges occurring in case of a short circuit first heats an enclosed gas volume. The resulting pressure wave acts upon a movable piston and carries out the mechanical contact opening work on it.
One advantageous feature of the invention is that through the use of a large-area resistive member with significantly higher electrical resistance compared to a metal, a localized melting of the electrodes is prevented on the one hand through the two-dimensional distribution of the current flow. On the other hand, a uniform heating of the gas chamber is encouraged.
The switch according to the invention is used in power distribution networks in the low-voltage area. In case of a disruption, particularly in case of short circuits, parts of the network must be switched away at higher-order branches. In order to limit or rather to avoid damages at the site of the disruption as well as in the area of the network, the disconnection should take place as quickly as possible, in particular, still within the first affected half cycle. In addition, limiting of the short-circuit current is also often required if the disconnection cannot be recognized quickly enough or rather cannot be carried out through suitable measures. The limiting of the short-circuit current also results in a limiting of the amplitudes o

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