Should I be obligated to conduct interviews when there's no compensation for the additional responsibility?

I am going to answer this in two, contradictory statements:

On your first question: Should you be obligated to XYZ....

No.

You are not obligated to do anything. That said there may be consequences for declining to do so.

But now let us answer your second question:

Is it standard practice for companies to expect engineers to conduct interviews without additional compensation?

Yes. You are on a Salary, if you have to take time out to do interviews, this is pretty common. Deadlines and timelines get pushed out a bit.

This is especially common in technical fields - where only people who work in the field can tell the difference between a BS Artist and someone who really knows their stuff.

Now, being a part of the interview process is highly advantageous. Firstly, there is a prestige associated with it.

Secondly - you get to add your input to the hiring process. When I have been involved in hiring, I prefer to test a candidates abstract reasoning. Knowledge can be learned, but someone's ability to think is something I value. I would rather someone who knows how and where to look for information - than someone who knows the answer to a specific question, but doesn't know how to research.

However!

I don't think this is really the question(s) you are asking:

I'm not being offered any additional compensation or bonus for taking it on (my salary has stayed the same for 8 years basically when I joined).

When I raised the compensation question with my HR head, they never responded.

Methinks if you had received regular yearly raises to keep up with Inflation, you would not be asking this question. The above - IMO is the real question you are asking.

You are unhappy with what you are being paid, rather than being unhappy with additional tasks.

And so my advise would be to sit down with your management and HR - outline the key highlights from your last 8 years of employment and what you have done to earn a raise. Find out the going rate for your years of experience and job title - present them with a figure (I recommend 10-20 percent higher than what you will settle for) and negotiate that.

Doing interviews are no different than your manager coming over and giving you more work.

If you're already busy, tell them which current responsibilities can be delayed to make room for the new priorities.

This is a fairly simple problem. Ask your manager what can be deprioritized to free up 3-4 hours to conduct the interviews.

Is it standard practice for companies to expect engineers to conduct interviews without additional compensation?

Yes. It's part of your normal duties.

I'm concerned that this extra workload will impact my primary responsibilities and professional development time.

Then discuss priorities with your management. Note that being asked to do interviews is a GOOD thing: Hiring is a very important decision and it seems that management trusts your judgement as an input to that decision. It's certainly one way to do professional development.

Basically why should I do something if I am not being fairly recognized/compensated for it?

You should not pout like a little kid, but identify the problem and start working on it. Outright refusing the ask is just dumb.

my salary has stayed the same for 8 years basically when I joined.

So here we have the problem. Your compensation has not been keeping up with your value and/or scope of responsibilities. There are lots of questions on this forum about "how to ask for a raise" but it boils down a few simple steps

1. Understand company policy on compensation management: salary bands, job titles, performance review process, bonus, promotion criteria, etc. Should be all in your handbook. Read it.
2. Estimate your market value by doing competitive analysis, online resources and if feasible getting alternative offers.
3. Carefully document you achievements, goal metrics, promotion metrics, etc against the criteria developed in step 1
4. Have a sit down with your boss and present the data that documents that you are underpaid. Propose a career development plan with actionable steps and clear metrics that would get you to the next step (promotion, target raise, new or expanded role, etc).
5. Either your boss engages constructively, then you have a plan to execute. Or they brush you off, in which case they are simply not particularly interested in developing you, so you should be looking elsewhere.

Again, you being asked to do interviews is a good sign, so chances are this will work. Add it to the data you will collect in step 3: it's another argument that you are underpaid.

Is it standard practice for companies to expect engineers to conduct interviews without additional compensation?

Normally, engineers should be allowed to allocate the interview hours in their work schedule of the week. For example, an engineer could report to his boss that he works 36 hours in a project, and spends 4 hours to interviews candidates (suppose that engineer works 40 hours per week).

Thus, in this case, you don't get paid extra for doing the interviews. But, those interview hours are counted toward your normal work hours of the week.

my salary has stayed the same for 8 years basically when I joined

Clearly, your company is not normal.

So, you should probably have looked for a new job after 2 consecutive years without a salary raise. You should definitely look for a new job now after 8 years.

I wouldn't refuse to do the interviews. Saying it's not part of your job description makes you come across as inflexible and uninvested in the success of the team. Work with your supervisor to figure out how to balance the interviews with your other priorities.

In addition to helping the company, taking on this additional responsibility will help you grow professionally and it should be met with recognition and reward but you can't expect to have that as a precondition. Raises and promotions generally come in response to things you've already done rather than what you commit to doing in the future. Keep track of all your accomplishments and the value they add so you can bring them up at your performance review.

If you feel the company is failing to recognize your achievements, then the optimal solution is to find a new job and resign. By refusing to do what's asked, you'll likely end up out of a job regardless so you might as well do the interviews and then start a conversation with your boss about career growth. If you come out of it feeling like the terms of your employment are no longer amenable, you can begin the transition to your next opportunity with dignity and professionalism. This is almost always better than the alternative.

Major Learning Opportunity

You will learn a lot by being on the other side of the interview table. You will get to see how other people interview, how they build their resumes, what skills they list, and how they conduct themselves. This is very important since it will improve your ability to interview which can be quite relevant since:

my salary has stayed the same for 8 years basically when I joined.

If you live a country that has experienced any form of inflation over the last 8 years, then your company has for all intents and purposes docked your pay. Considering other things you have mentioned in your comments, this company has demonstrated that they do not care about you, so why should you care about the company and its image? If the client gets upset that you were not present to provide support, is that your fault, or your management's fault for choosing to ignore their employee who has reported a schedule conflict?

As such you should be seeing what other job opportunities are out there and doing your own interviewing while you still have a paycheck coming in. Also be sure to take your current salary adjust for 8 years of inflation and small pay raises to give yourself an idea of the minimum salary you should be looking for.

I am a bit surprised and baffled that your company would even want you to participate in these interviews considering how they have treated you. There is phrase that is thrown around: Interviewing is a two way street. There would be nothing stopping you from hinting to or commenting on your experiences with the company during the interview which would cause candidates to rethink accepting any job offer made.

In conclusion (TL;DR): Follow your management's instructions and do the interviews, learn as much as you can from conducting them, and apply it to your own inevitable job searching.

After 8 years with no rise in pay, the only interviews you should be doing, should be as a candidate!

You'd do better to separate your demands for more money, from the question of whether you're willing to help with interviewing.

Generally speaking, those who are not managers of staff are not expected or compelled to conduct interviews, and it is something you do only if you wish to do when offered.

It's not a particularly valuable activity to the employer in its own right - it's usually done just to wage a war of attrition with over-supplied candidates, and to avoid having to offer up a real manager for the initial round of interview - so it's not something that a company would normally want to increase your pay for (although it might offer time-and-half for something outside normal hours).

It's often a handy experience for the staff member who does want to proceed to people management, so the only reason to turn it down would be either if you lack the confidence or interest to interact with candidates (and have no aspirations to people management), or if you hate your employer so much at this stage that you don't feel you can make a reasonable impression.

If you want a payrise, ask for it separately.

Should I be obligated to conduct interviews when there's no compensation for the additional responsibility?

No, but there are drawbacks trying to get a better deal.

1. If you say "No" to the manager, he will not be happy. Not good for you.
2. You do not have the right to complain about employees being of insufficient quality - after all, you do not contribute anything. You might be the one to actually filter candidates better - but you prefer to decline.
3. If you want to be paid more for the job of interviewing (preparing for the interview, running it, preparing answer), hen you definitely need to be paid less of the normal salary (when you work for the interview, you do not do productive work for the projects). Do you think that the interview work pays more than the actual work you do?
4. You miss the opportunity of promotion - either technical, or administrative.

Now, think and choose wisely :)

While there are a lot of good answers that address part of the question, I do think some have missed a subtlety here.

To a reasonable order of approximation, all salaried jobs involve you getting paid to perform duties within your working hours.

However, people are not all paid the same. Roles that have "greater responsibilities" that others generally get paid more.

So the question of "should I be getting a pay raise since I'm being asked to conduct interviews?" has some logic to it, depending on how you interpreter "responsibilities".

For many engineers, conducting an interview simply means that you are talking to candidates and providing your professional opinion on their suitability for a role. This is not a responsibility. This is a "task" or "duty".

You are not being held responsible for making a hiring decision. You are being asked to provide you professional opinion on a matter that you are qualified to opine on. This falls well within the scope of your day-to-day role.

If you were being asked to make hiring decisions, then it would be reasonable to request an increase in salary, since the impact of that role is more significant and the value that you're providing to the company goes beyond your day-to-day duties.

So, to answer your question: if you're just being asked to conduct interviews, then you are obliged to perform this task to the extent that you're obliged to do any of your other day-to-day tasks. If you're being asked to make hiring decisions, then you should be discussing an increase in compensation commensurate with the increase in value that you're providing your employer.

A little off-topic, if you haven't had a raise in 8 years, check the market, perhaps you are being underpaid

Candidate screening, research and interview can give you a bit of insides regarding going rates