The article I linked included:
Other known effects of Test 184 were that it knocked out a major 1000-kilometer (600-mile) underground power line running from Astana (then called Aqmola), now the capital city of Kazakhstan, to the city of Almaty. Some fires were reported. In the city of Karaganda, the EMP started a fire in the city's electrical power plant, which was connected to the long underground power line. The shielded electrical cable was buried 3 feet (90 cm.) underground. The geomagnetic-storm-like E3 component of the EMP (also called MHD-EMP) can easily penetrate into the ground. The E3 component of the Test 184 detonation (caused by the movement of the Earth's magnetic field) began rising immediately after the detonation, but did not reach its peak until 20 seconds after the detonation. The E3 pulse then decayed over the next minute or so. The E3 component only affects equipment connected to long electrical conductors.
So, the answer is that even shielded cables, unless buried very deeply, are vulnerable to EMP.
That’s interesting. I had thought that military bases burying of electrical lines was to protect them from EMP. Guess not, or if they were attempting to do that, I guess it won’t work anyway.