Technical FAQs

Ask a Question

Why is Isolating or Electrostatic Shielding (IS) not an option on the new DOE 2016 EX type Three Phase Transformers?

Electrostatic Shields on EX Transformers

Product Line:
LV Transformers

Applies to EX series Low Voltage Transformers by SquareD/Schneider Electric

Prior Model EE series Transformers did have an option for Electrostatic Shielding


Shields were a 1970s theoretical “solution” to the hash and intermittent spikes on building electrical systems. There were no 600 Volt MOVs or TVSS at the time.
The theory went that the increase of interwinding capacitance provided by a shield would attenuate the frequencies that a Fourier analysis showed existed in a sampling of the spikes. Evaluations of shield attenuation to validate the theory were made using timed pulses of DC signal impressed on the primary to model the spikes and hash. It is unclear if there were tests done on unshielded units as a control.
The attenuation values shown below are taken straight from our old Shielded transformer specification and have been adopted by the industry for years.
It was found a while ago that unshielded units would meet those levels if the unit was tested under load with real AC frequencies to simulate the spikes and hash.  The attenuation seems to be more a function of the transformers reactance (inductive and capacitive) than the capacitive nature of the shields.
If the customer has a truly horrendous noise issue, there are better methods to address it than offering a shielded transformer. TVSS or wave shaping equipment are readily available Schneider Electric products, most transformer companies can’t offer them and so they continue to offer shields.
The link below directs you to our SURGE PROTECTION AND POWER CONDITIONING page.
Square D unshielded isolation transformers will meet the following attenuation levels when grounded in compliance with the NEC Articles 250 and 450.
Common Mode:      0 to 1.5kHZ - 120dB; 1.5kHZ to 10kHZ - 90dB; 10kHZ to 100kHZ - 65dB; 100kHZ to 1MHZ - 40dB
Transverse Mode:  1.5kHZ to 10kHZ - 52dB; 10kHZ to 100kHZ - 30dB; 100kHZ to 1MHZ –30dB
Was this helpful?
What can we do to improve the information ?