Switch Data

Switch Data

Two standard types of switch are available within PBwel Buchholz Relays, Mercury (wetted contact) switches for use in standard applications or magnetically operated Reed (dry contact) switches, originally designed for areas with seismic activity but now more commonly employed, due to their performance and restrictions upon the use and transportation of Mercury.

Mercury Switches

Mercury switches are employed of a special design to prevent mal-operation due to excessive transformer vibration. A sample relay of this type has been submitted to a continuous 3000 hour vibratory type test. During this test the relay was vibrated to an amplitude of 0.01 in.  (0.254mm)  peak to peak at a frequency of 100 Hz. The mercury switches were connected to sensitive detecting equipment and no mal-operations were recorded.

The mercury switches are spring mounted within the switch cylinders and protected from possible damage. Alarm and Trip circuit mercury switches will make, break and carry continuously 2 Amps at 250 Volts A.C or D.C. They will also make and carry 10 Amps at 250 Volts A.C. or D.C. for 0.5 second.

Reed Switches

Shock and vibration acting along the tube of a conventional mercury switch can cause the mercury within it to move and momentarily bridge the switch electrodes, even though the switch is tilted in the open position. This is considered to be a mal-operation of the relay in that it is caused by externa! influences and not by a fault within the transformer. Consequently, where relays are to be used in situations subject to seismic disturbance or blasting, a more suitable alternative to the usual mercury switch is required and magnet operated reed switches are available specifically for this purpose. These switches have Rhodium contacts, located midway along the length of their glass tubes. The tubes contain an atmosphere of Nitrogen.

Connected in series with each reed switch, and mounted within the terminal box, is an inductor of approximately 30 microhenries and 0.04ohm. These inductors are intended to protect the reed switch contacts from the effects of capacitive loads, such as are imposed by long leads or pilot cables, and must not be removed from relays in service.

Protection of reed switch contacts against the effect of inductive loads, such as are imposed by Tripping Relays is achieved by means of a diode wired across each load. This diode must be rated with forward current at least as high as the steady load current and connected observing polarity so as to absorb the back e.m.f. Diode Protection units fitted with suitably rated diodes for this purpose in available. (see Diodes Unit)

Reed Switch Data

Single Contact Change-over Contact
Type
Gunther Type 1526
Gunther Type 1621
Switch Capacity
Max. 250 VA/W
Max. 60 W/80VA
Switching Current
Max. 5A
Max. 2A
Switching Voltage (0-60Hz)
Max. 250 V
Max. 220V
Initial Contact Resistance
Max. 100 milliohms
Max. 100 milliohms
Breakdown Voltage
Min. 600 V r.m.s.
500/400 V d.c.
Resonance Frequency
900Hz
-
Shock Resistance
Max. 50g (duration 11ms)
Max. 50g (duration 11ms)
Vibration Resistance
Max. 35g (50-500Hz)
Max. 35g (50-500Hz)
Temperature range
— 55°C to + 150°C
— 40°C to +50°C

Ability to withstand vibration due to Earth Tremors and Blasting

A device, with its contacts electronically monitored (by means of an instrument capable of registering and recording a contact closure of 1ms duration), was subjected to a sinusoidal vibration at a frequency of 100 Hz and an amplitude of 0.25” (0.05 mm) peak to peak (thus a maximum acceleration of 6g) in the plane of movement of the contact making arrangement for a period of 1000 hours, during which there was no recorded operation of the contacts.

Immediately before, and again immediately after the vibration test, the stability of the device and its contacts under earth tremor conditions was proven by subjecting the device, whilst being vibrated under the conditions of the vibration test above, to further vibrations superimposed on the 100Hz vibration and supplied separately in each of the three perpendicular axes one of which was in the same plane as the 100Hz vibration.

These vibrations had a constant peak to peak amplitude of 2.5mm and were carried by a continuous slow sweep over the range of 0.1 to 33Hz (at which frequency the maximum acceleration was 5.5g) in order to search out resonances. Again no operation of the Buchholz was found.

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