A P10 should be able to get into any average mundane's mind. A telepath may be able to keep them out for a while.
http://www.midwinter.com/lurk/guide/000.html
JMS Speaks:
"The amount of contact required varies according to the telepath's strength. Lyta at P5 needs a little help. A P10 could nail you from across the room."
JMS Speaks:
http://www.midwinter.com/lurk/guide/006.html
"A Psi-rating comes through training and examination of a person's skills over time. Ivanova's mother never went through the full sequence to get rated. (Although they generally don't bother with P1s through P2s, so she was at least a P3 or above, in terms of raw ability.) A psi rating isn't hereditary."
Plot Points
http://www.midwinter.com/lurk/guide/101.html
"The Corps is training some of its members in offensive skills known as "mind shredding." A P10-rated shredder is a danger even to a P12-rated telepath, and a P12-rated shredder can kill a mundane telepathically in a matter of seconds."
"Psi Corps field operatives must be able to maintain a mental block against a telepathic probe for at least one hour to ensure that they're safe against attack by untrained rogues. Even the most powerful rogues, according to Bester, can't sustain a psi attack for more than 30 or 40 minutes."
So according to this and the show...the rules need some help. I am more than willing to help :!: