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6. Leakage Currents
An important distinction in medical electrical equipment.
Class I describes equipment protected through a protective earth connection.
B, BF and CF are separate classifications relating to PATIENT-APPLIED PARTS, with
CF having particularly stringent patient-leakage current limits.
Protective-earth leakage and patient leakage are different measurements, with different limits.
Feedback highlighted that an explanation could make this distinction clearer.
A useful reminder to identify precisely WHICH leakage current a question is testing.
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4. Normal Distribution
A useful reminder of the familiar 68–95–99.7 rule.
For a normal distribution:
• 68% of observations lie within ±1 SD
• 95% within ±2 SD
• 99.7% within ±3 SD
Therefore, approximately 0.3% lie OUTSIDE ±3 SD across BOTH tails.
Only approximately 0.15% lie beyond 3 SD in ONE direction.
This simple approach is generally sufficient for Primary FRCA questions.
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3. Electrical Safety
What defines Class II electrical equipment?
Class II equipment relies on DOUBLE or REINFORCED insulation rather than protective earthing.
It is identified by the double-square symbol.
This differs from Class I equipment, which uses a protective earth connection, and Class III equipment, which operates at safety extra-low voltage.
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2. Shunt & Oxygen
At what shunt fraction does hypoxaemia become refractory to 100% oxygen?
A figure of approximately 30% is sometimes quoted, but this is a teaching approximation rather than a strict physiological threshold.
As shunt fraction increases, the effectiveness of increasing FiO₂ progressively decreases.
Even with substantial shunting, increasing FiO₂ may still produce some improvement in arterial oxygenation.
The key principle is that shunted blood bypasses ventilated alveoli, limiting the benefit of increasing inspired oxygen.
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Round 2 - Here are the latest learning points from queries raised by users of Oxford FRCA, both in Discuss and via the app feedback.
1. Ultrasound frequency and penetration
Why might a 3 MHz ultrasound probe be selected for imaging deep structures such as the kidneys?
The answer is the compromise between PENETRATION and RESOLUTION.
Lower-frequency ultrasound undergoes less attenuation, improving penetration into deeper tissues.
However, higher-frequency ultrasound provides better spatial resolution.