What is an electrostatic shield?

The electrostatic shield is simply a grounded single turn of conductive nonferrous foil placed between coils to divert primary noise to ground.The conductive foil completely enclosing the windings will provide a ground path for primary circuit noise and has the advantage that a very much smaller capacitance exists between primary and secondary coils than in the case of a simple Faraday shield. The enclosing shield will not re-radiate the noise signal, and will provide effective electromagnetic noise reduction. Typically, according to Topaz at a distance of 18 inches from a transformer's geometric center, the field strength will be less than 0.1 gauss, and will roughly follow inverse cube laws.

Isolation transformers with electrostatic shields are used for power supplies for sensitive equipment such as computers or laboratory instruments. An isolation transformer is designed to address the problems associated with referencing its internal shields to ground. It is constructed with two isolated Faraday shields between the primary and secondary windings. The shield, which is closest to the primary winding, is connected to the common power supply ground and the shield closest to the secondary winding is connected to the shield of the circuit to be isolated. The use of two shields in the construction of the isolation transformer diverts high frequency noise, which would normally be coupled across the transformer to the grounds of the circuit in which they occur. The two shields provide more effective isolation of the primary and secondary circuits by also isolating their grounds.
The isolation transformer adds a third capacitance between the two Faraday shields, which may allow coupling of high frequency noise between the system grounds. However, increasing the separation between the two Faraday shields normally minimizes this third capacitance. Additionally, the dielectric effect of the shields plus the increased separation of the windings significantly reduce the inter-capacitance between the windings.

Since inter-winding capacitance is the primary path by which significant power line and transient related noise couples to the system, more information is needed to describe what occurs. During the time power is being transferred between transformer windings, noise potentials between the primary circuits and ground is similarly coupled to the secondary through both capacitive and resistive paths. This noise appears in three forms normally in a transformer circuit: common-mode, transverse mode, and electromagnetic.

Common - Mode Noise

This noise appears between both sides of a power line and ground. Since this noise is referenced to the power system ground, the most obvious method of eliminating this noise is by grounding the transformer center tap to the system ground via the lowest impedance path possible. Internal transformer designs, which separate the coils to reduce capacitive coupling, have some advantage, but it also increases leakage inductance and reduces the power transfer.

Transverse - Mode

Transverse-mode noise is much more difficult to eliminate than common-mode noise. The key here is to differentiate between power and noise, and then reduce the noise. Noise and power are separated by the difference in their frequencies. The most effective transformer would be a design exactly opposite to a audio transformer. The purpose is to transfer the power required by the load at the fundamental power frequency and to eliminate all higher and lower frequencies. Sub-harmonic frequencies are attenuated by operating the transformer at relatively high flux density, which is effective in reducing or eliminating them. Above the fundamental frequency, noise is reduced by introducing as much leakage inductance as possible, consistent with good power transfer to the secondary.
Transverse-mode noise appears as a voltage across both the primary and secondary windings of an isolation transformer. It occurs when a common-mode noise signal causes current to flow in the primary winding (or secondary winding), and from there to ground via capacitance to a grounded shield. Common-mode noise can also be transformed into 'transverse-mode noise, and thereby, through magnetic coupling, contaminate the secondary of an isolation transformer. Normally, by the proper selection of core loss verses primary winding inductance, a well-designed isolation transformer will eliminate the majority of this type of noise. Here again, grounding the transformer shield to the lowest impedance path available, will result in noise currents using this return path rather than some other higher impedance path to the noise source ground.

Electromagnetic Noise

Electromagnetic noise does not constitute a major problem in most applications, but is sometimes critical in some recording or digital data systems, and in making electromagnetic interference measurements.

Rack Level Applications

The most effective application of isolation transformers is with racks of equipment. A rack acts as an outer shield for internal instruments, while serving as the zero-signal reference for system output signals. Isolation transformers are used to control shield currents, and to break up the mutual capacitance between rack instrumentation and an unknown power ground. The main benefit of using an isolation transformer with a rack of equipment is the enhanced control of currents in the equipment shields. Any potential differences between the utility power ground and the rack's ground will cause currents to flow in the loop. The isolation transformer allows these "ground" currents to be directed through a portion of the rack's shielding which will not effect the operation of sensitive circuits and completely isolates these currents from the internal equipment reference conductors.

Room Level Applications

It is often necessary to isolate EMC test enclosures from noisy building grounds. Not only can isolation transformers be used to effectively decouple building power, but also since they also act as tuned circuits; they reduce the differential noise from external equipment, which reaches your screen room. While it is recognized as a second isolation transformer inside the test room will greatly reduce power line ambient, this section will only consider using transformers on the power lines to a typical screen room. As with any transformer, isolation transformers radiate magnetic fields. Physically locating the transformer adjacent to, or connected to, a screen room may increase rather than decrease ambient noise. Since the physical case of a transformer, as well as the primary winding shield, are normally connected to the third-wire power ground of the supplied power, the secondary winding shield must be isolated from the transformer case and connected only to the conduit shield going to the shielded room to achieve proper ground isolation. The conduit acts as an RF shield for the room's power and completes the connection between the shielded room and the secondary winding shield in the transformer. If the transformer is three phase and supplies more than one room, the best application for isolation between rooms is to use only one phase for each room, with a limit of three rooms per transformer. With this approach, power line filters will effectively isolate the room while providing practical noise attenuation. Proper transformer design, wiring, and, above all, grounding, are the only effective means of reducing the three types of noise problems. Grounding should be controlled and use the lowest impedance path possible (i.e., bonding) to the central reference ground system to insure maximum attenuation of noise sources. To achieve the maximum protection from a transformer, not only must it be applied properly, but also the transformer should be one specially designed for isolation usage. The Shielded three phase isolation transformers have all the feature of the standard 3 phase plus they also incorporate a full metallic shield (usually copper or aluminum) between the 3 phase primary and 3 phase secondary windings. This electrostatic shield or Faraday Shield, is connected to earth ground and performs two functions: Its attenuates (filters) voltage transients (voltage spikes). These shielded 3 phase isolation transformers have an attenuation ratio of 100 to 1. It filters common mode noise, Attenuation of approximately 30 decibels. The shield three phase isolation transformer is preferred over the standard three phase isolation transformer because it provides protection to sensitive and critical equipment. When more that one shielded 3 phase isolation transformer is used between the source and the load, it is referred to as a " cascading" and greatly improves power quality.

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