This is what IEEE 802.3 requires(for 100BaseT--the requirements for 10BaseT are identical and covered in 14.3.1.1).
Who knows whether the equipment involved here actually meets these specifications?
23.5.1.1 Isolation requirement
The PHY shall provide electrical isolation between the DTE, or repeater circuits including frame ground,
and all MDI leads. This electrical separation shall withstand at least one of the following electrical strength
tests:
a) 1500 V rms at 50 Hz to 60 Hz for 60 s, applied as specified in subclause 5.3.2 of IEC 60950: 1991.
b) 2250 Vdc for 60 s, applied as specified in subclause 5.3.2 of IEC 60950: 1991.
c) A sequence of ten 2400 V impulses of alternating polarity, applied at intervals of not less than 1 s.
The shape of the impulses shall be 1.2/50 ìs (1.2 ìs virtual front time, 50 ìs virtual time or half
value), as defined in IEC 60060.
There shall be no insulation breakdown, as defined in subclause 5.3.2 of IEC 60950: 1991, during the test.
The resistance after the test shall be at least 2 MÙ, measured at 500 Vdc.
